diff --git a/crates/objc2/CHANGELOG.md b/crates/objc2/CHANGELOG.md
index ce13dae31..931643621 100644
--- a/crates/objc2/CHANGELOG.md
+++ b/crates/objc2/CHANGELOG.md
@@ -22,6 +22,10 @@ The format is based on [Keep a Changelog](https://keepachangelog.com/en/1.0.0/).
 * Added `Allocated::set_ivars`, which sets the instance variables of an
   object, and returns the new `rc::PartialInit`.
 * Added the ability for `msg_send_id!` to call `super` methods.
+* Implement `Send` and `Sync` for `ProtocolObject` if the underlying protocol
+  implements it.
+* Added ability to create `Send` and `Sync` versions of
+  `ProtocolObject<dyn NSObjectProtocol>`.
 
 ### Changed
 * **BREAKING**: Changed how instance variables work in `declare_class!`.
diff --git a/crates/objc2/src/macros/extern_protocol.rs b/crates/objc2/src/macros/extern_protocol.rs
index 9771871cb..1a99770a0 100644
--- a/crates/objc2/src/macros/extern_protocol.rs
+++ b/crates/objc2/src/macros/extern_protocol.rs
@@ -204,6 +204,9 @@ macro_rules! __inner_extern_protocol {
         {
             const __INNER: () = ();
         }
+
+        // TODO: Should we also implement `ImplementedBy` for `Send + Sync`
+        // types, as is done for `NSObjectProtocol`?
     };
 }
 
diff --git a/crates/objc2/src/runtime/nsobject.rs b/crates/objc2/src/runtime/nsobject.rs
index 3672a5c85..deb48c856 100644
--- a/crates/objc2/src/runtime/nsobject.rs
+++ b/crates/objc2/src/runtime/nsobject.rs
@@ -7,6 +7,8 @@ use crate::runtime::{AnyClass, AnyObject, ProtocolObject};
 use crate::{extern_methods, msg_send, msg_send_id, Message};
 use crate::{ClassType, ProtocolType};
 
+use super::ImplementedBy;
+
 /// The root class of most Objective-C class hierarchies.
 ///
 /// This represents the [`NSObject` class][cls]. The name "NSObject" also
@@ -163,6 +165,33 @@ crate::__inner_extern_protocol!(
     ("NSObject")
 );
 
+// SAFETY: Anything that implements `NSObjectProtocol` and is `Send` is valid
+// to convert to `ProtocolObject<dyn NSObjectProtocol + Send>`.
+unsafe impl<T> ImplementedBy<T> for dyn NSObjectProtocol + Send
+where
+    T: ?Sized + Message + NSObjectProtocol + Send,
+{
+    const __INNER: () = ();
+}
+
+// SAFETY: Anything that implements `NSObjectProtocol` and is `Sync` is valid
+// to convert to `ProtocolObject<dyn NSObjectProtocol + Sync>`.
+unsafe impl<T> ImplementedBy<T> for dyn NSObjectProtocol + Sync
+where
+    T: ?Sized + Message + NSObjectProtocol + Sync,
+{
+    const __INNER: () = ();
+}
+
+// SAFETY: Anything that implements `NSObjectProtocol` and is `Send + Sync` is
+// valid to convert to `ProtocolObject<dyn NSObjectProtocol + Send + Sync>`.
+unsafe impl<T> ImplementedBy<T> for dyn NSObjectProtocol + Send + Sync
+where
+    T: ?Sized + Message + NSObjectProtocol + Send + Sync,
+{
+    const __INNER: () = ();
+}
+
 unsafe impl NSObjectProtocol for NSObject {}
 
 extern_methods!(
diff --git a/crates/objc2/src/runtime/protocol_object.rs b/crates/objc2/src/runtime/protocol_object.rs
index 62a9b40d1..384ebf99c 100644
--- a/crates/objc2/src/runtime/protocol_object.rs
+++ b/crates/objc2/src/runtime/protocol_object.rs
@@ -62,6 +62,16 @@ pub struct ProtocolObject<P: ?Sized> {
     p: PhantomData<P>,
 }
 
+// SAFETY: `Send` if the underlying trait promises `Send`.
+//
+// E.g. `ProtocolObject<dyn NSObjectProtocol + Send>` is naturally `Send`.
+unsafe impl<P: ?Sized + Send> Send for ProtocolObject<P> {}
+
+// SAFETY: `Sync` if the underlying trait promises `Sync`.
+//
+// E.g. `ProtocolObject<dyn NSObjectProtocol + Sync>` is naturally `Sync`.
+unsafe impl<P: ?Sized + Sync> Sync for ProtocolObject<P> {}
+
 // SAFETY: The type is `#[repr(C)]` and `AnyObject` internally
 unsafe impl<P: ?Sized> RefEncode for ProtocolObject<P> {
     const ENCODING_REF: Encoding = Encoding::Object;
@@ -264,6 +274,9 @@ mod tests {
     unsafe impl FooBar for DummyClass {}
     // unsafe impl FooFooBar for DummyClass {}
 
+    unsafe impl Send for DummyClass {}
+    unsafe impl Sync for DummyClass {}
+
     extern_methods!(
         unsafe impl DummyClass {
             #[method_id(new)]
@@ -275,6 +288,12 @@ mod tests {
     fn impl_traits() {
         assert_impl_all!(NSObject: NSObjectProtocol);
         assert_impl_all!(ProtocolObject<dyn NSObjectProtocol>: NSObjectProtocol);
+        assert_not_impl_any!(ProtocolObject<dyn NSObjectProtocol>: Send, Sync);
+        assert_impl_all!(ProtocolObject<dyn NSObjectProtocol + Send>: NSObjectProtocol, Send);
+        assert_not_impl_any!(ProtocolObject<dyn NSObjectProtocol + Send>: Sync);
+        assert_impl_all!(ProtocolObject<dyn NSObjectProtocol + Sync>: NSObjectProtocol, Sync);
+        assert_not_impl_any!(ProtocolObject<dyn NSObjectProtocol + Sync>: Send);
+        assert_impl_all!(ProtocolObject<dyn NSObjectProtocol + Send + Sync>: NSObjectProtocol, Send, Sync);
         assert_not_impl_any!(ProtocolObject<dyn Foo>: NSObjectProtocol);
         assert_impl_all!(ProtocolObject<dyn Bar>: NSObjectProtocol);
         assert_impl_all!(ProtocolObject<dyn FooBar>: NSObjectProtocol);
@@ -332,6 +351,10 @@ mod tests {
         let _nsobject: &ProtocolObject<dyn NSObjectProtocol> = ProtocolObject::from_ref(bar);
         let nsobject: &ProtocolObject<dyn NSObjectProtocol> = ProtocolObject::from_ref(&*obj);
         let _nsobject: &ProtocolObject<dyn NSObjectProtocol> = ProtocolObject::from_ref(nsobject);
+        let _: &ProtocolObject<dyn NSObjectProtocol + Send> = ProtocolObject::from_ref(&*obj);
+        let _: &ProtocolObject<dyn NSObjectProtocol + Sync> = ProtocolObject::from_ref(&*obj);
+        let _: &ProtocolObject<dyn NSObjectProtocol + Send + Sync> =
+            ProtocolObject::from_ref(&*obj);
 
         let _foobar: &mut ProtocolObject<dyn FooBar> = ProtocolObject::from_mut(&mut *obj);
         let _foobar: Id<ProtocolObject<dyn FooBar>> = ProtocolObject::from_id(obj);
diff --git a/crates/test-ui/ui/protocol_object_only_protocols.stderr b/crates/test-ui/ui/protocol_object_only_protocols.stderr
index c337ea52c..038ea3af0 100644
--- a/crates/test-ui/ui/protocol_object_only_protocols.stderr
+++ b/crates/test-ui/ui/protocol_object_only_protocols.stderr
@@ -8,13 +8,13 @@ error[E0277]: the trait bound `NSObject: ImplementedBy<NSObject>` is not satisfi
   |
   = help: the following other types implement trait `ImplementedBy<T>`:
             (dyn NSObjectProtocol + 'static)
+            (dyn NSObjectProtocol + Send + 'static)
+            (dyn NSObjectProtocol + Sync + 'static)
+            (dyn NSObjectProtocol + Send + Sync + 'static)
             (dyn NSAccessibilityColor + 'static)
             (dyn NSAccessibilityCustomRotorItemSearchDelegate + 'static)
             (dyn NSAccessibilityElementProtocol + 'static)
             (dyn NSAccessibilityGroup + 'static)
-            (dyn NSAccessibilityButton + 'static)
-            (dyn NSAccessibilitySwitch + 'static)
-            (dyn NSAccessibilityRadioButton + 'static)
           and $N others
 note: required by a bound in `ProtocolObject::<P>::from_ref`
  --> $WORKSPACE/crates/objc2/src/runtime/protocol_object.rs
@@ -35,13 +35,13 @@ error[E0277]: the trait bound `dyn Send: ImplementedBy<NSObject>` is not satisfi
   |
   = help: the following other types implement trait `ImplementedBy<T>`:
             (dyn NSObjectProtocol + 'static)
+            (dyn NSObjectProtocol + Send + 'static)
+            (dyn NSObjectProtocol + Sync + 'static)
+            (dyn NSObjectProtocol + Send + Sync + 'static)
             (dyn NSAccessibilityColor + 'static)
             (dyn NSAccessibilityCustomRotorItemSearchDelegate + 'static)
             (dyn NSAccessibilityElementProtocol + 'static)
             (dyn NSAccessibilityGroup + 'static)
-            (dyn NSAccessibilityButton + 'static)
-            (dyn NSAccessibilitySwitch + 'static)
-            (dyn NSAccessibilityRadioButton + 'static)
           and $N others
 note: required by a bound in `ProtocolObject::<P>::from_ref`
  --> $WORKSPACE/crates/objc2/src/runtime/protocol_object.rs
@@ -62,13 +62,13 @@ error[E0277]: the trait bound `dyn Foo: ImplementedBy<NSObject>` is not satisfie
   |
   = help: the following other types implement trait `ImplementedBy<T>`:
             (dyn NSObjectProtocol + 'static)
+            (dyn NSObjectProtocol + Send + 'static)
+            (dyn NSObjectProtocol + Sync + 'static)
+            (dyn NSObjectProtocol + Send + Sync + 'static)
             (dyn NSAccessibilityColor + 'static)
             (dyn NSAccessibilityCustomRotorItemSearchDelegate + 'static)
             (dyn NSAccessibilityElementProtocol + 'static)
             (dyn NSAccessibilityGroup + 'static)
-            (dyn NSAccessibilityButton + 'static)
-            (dyn NSAccessibilitySwitch + 'static)
-            (dyn NSAccessibilityRadioButton + 'static)
           and $N others
 note: required by a bound in `ProtocolObject::<P>::from_ref`
  --> $WORKSPACE/crates/objc2/src/runtime/protocol_object.rs
@@ -79,24 +79,52 @@ note: required by a bound in `ProtocolObject::<P>::from_ref`
   |         P: ImplementedBy<T>,
   |            ^^^^^^^^^^^^^^^^ required by this bound in `ProtocolObject::<P>::from_ref`
 
-error[E0277]: the trait bound `dyn NSObjectProtocol + Send: ImplementedBy<NSObject>` is not satisfied
+error[E0277]: `*const UnsafeCell<()>` cannot be sent between threads safely
  --> ui/protocol_object_only_protocols.rs
   |
   |     let _: &ProtocolObject<dyn NSObjectProtocol + Send> = ProtocolObject::from_ref(&*obj);
-  |                                                           ------------------------ ^^^^^ the trait `ImplementedBy<NSObject>` is not implemented for `dyn NSObjectProtocol + Send`
+  |                                                           ------------------------ ^^^^^ `*const UnsafeCell<()>` cannot be sent between threads safely
   |                                                           |
   |                                                           required by a bound introduced by this call
   |
+  = help: within `NSObject`, the trait `Send` is not implemented for `*const UnsafeCell<()>`
   = help: the following other types implement trait `ImplementedBy<T>`:
             (dyn NSObjectProtocol + 'static)
+            (dyn NSObjectProtocol + Send + 'static)
+            (dyn NSObjectProtocol + Sync + 'static)
+            (dyn NSObjectProtocol + Send + Sync + 'static)
             (dyn NSAccessibilityColor + 'static)
             (dyn NSAccessibilityCustomRotorItemSearchDelegate + 'static)
             (dyn NSAccessibilityElementProtocol + 'static)
             (dyn NSAccessibilityGroup + 'static)
-            (dyn NSAccessibilityButton + 'static)
-            (dyn NSAccessibilitySwitch + 'static)
-            (dyn NSAccessibilityRadioButton + 'static)
           and $N others
+  = note: required because it appears within the type `(*const UnsafeCell<()>, PhantomPinned)`
+note: required because it appears within the type `PhantomData<(*const UnsafeCell<()>, PhantomPinned)>`
+ --> $RUST/core/src/marker.rs
+  |
+  | pub struct PhantomData<T: ?Sized>;
+  |            ^^^^^^^^^^^
+note: required because it appears within the type `UnsafeCell<PhantomData<(*const UnsafeCell<()>, PhantomPinned)>>`
+ --> $RUST/core/src/cell.rs
+  |
+  | pub struct UnsafeCell<T: ?Sized> {
+  |            ^^^^^^^^^^
+note: required because it appears within the type `objc_object`
+ --> $WORKSPACE/crates/objc-sys/src/object.rs
+  |
+  | pub struct objc_object {
+  |            ^^^^^^^^^^^
+note: required because it appears within the type `AnyObject`
+ --> $WORKSPACE/crates/objc2/src/runtime/mod.rs
+  |
+  | pub struct AnyObject(ffi::objc_object);
+  |            ^^^^^^^^^
+note: required because it appears within the type `NSObject`
+ --> $WORKSPACE/crates/objc2/src/runtime/nsobject.rs
+  |
+  | pub struct NSObject {
+  |            ^^^^^^^^
+  = note: required for `dyn NSObjectProtocol + Send` to implement `ImplementedBy<NSObject>`
 note: required by a bound in `ProtocolObject::<P>::from_ref`
  --> $WORKSPACE/crates/objc2/src/runtime/protocol_object.rs
   |
@@ -106,24 +134,85 @@ note: required by a bound in `ProtocolObject::<P>::from_ref`
   |         P: ImplementedBy<T>,
   |            ^^^^^^^^^^^^^^^^ required by this bound in `ProtocolObject::<P>::from_ref`
 
-error[E0277]: the trait bound `dyn NSObjectProtocol + Sync: ImplementedBy<NSObject>` is not satisfied
+error[E0277]: `UnsafeCell<PhantomData<(*const UnsafeCell<()>, PhantomPinned)>>` cannot be shared between threads safely
  --> ui/protocol_object_only_protocols.rs
   |
   |     let _: &ProtocolObject<dyn NSObjectProtocol + Sync> = ProtocolObject::from_ref(&*obj);
-  |                                                           ------------------------ ^^^^^ the trait `ImplementedBy<NSObject>` is not implemented for `dyn NSObjectProtocol + Sync`
+  |                                                           ------------------------ ^^^^^ `UnsafeCell<PhantomData<(*const UnsafeCell<()>, PhantomPinned)>>` cannot be shared between threads safely
   |                                                           |
   |                                                           required by a bound introduced by this call
   |
+  = help: within `NSObject`, the trait `Sync` is not implemented for `UnsafeCell<PhantomData<(*const UnsafeCell<()>, PhantomPinned)>>`
+  = help: the following other types implement trait `ImplementedBy<T>`:
+            (dyn NSObjectProtocol + 'static)
+            (dyn NSObjectProtocol + Send + 'static)
+            (dyn NSObjectProtocol + Sync + 'static)
+            (dyn NSObjectProtocol + Send + Sync + 'static)
+            (dyn NSAccessibilityColor + 'static)
+            (dyn NSAccessibilityCustomRotorItemSearchDelegate + 'static)
+            (dyn NSAccessibilityElementProtocol + 'static)
+            (dyn NSAccessibilityGroup + 'static)
+          and $N others
+note: required because it appears within the type `objc_object`
+ --> $WORKSPACE/crates/objc-sys/src/object.rs
+  |
+  | pub struct objc_object {
+  |            ^^^^^^^^^^^
+note: required because it appears within the type `AnyObject`
+ --> $WORKSPACE/crates/objc2/src/runtime/mod.rs
+  |
+  | pub struct AnyObject(ffi::objc_object);
+  |            ^^^^^^^^^
+note: required because it appears within the type `NSObject`
+ --> $WORKSPACE/crates/objc2/src/runtime/nsobject.rs
+  |
+  | pub struct NSObject {
+  |            ^^^^^^^^
+  = note: required for `dyn NSObjectProtocol + Sync` to implement `ImplementedBy<NSObject>`
+note: required by a bound in `ProtocolObject::<P>::from_ref`
+ --> $WORKSPACE/crates/objc2/src/runtime/protocol_object.rs
+  |
+  |     pub fn from_ref<T: ?Sized + Message>(obj: &T) -> &Self
+  |            -------- required by a bound in this associated function
+  |     where
+  |         P: ImplementedBy<T>,
+  |            ^^^^^^^^^^^^^^^^ required by this bound in `ProtocolObject::<P>::from_ref`
+
+error[E0277]: `UnsafeCell<PhantomData<(*const UnsafeCell<()>, PhantomPinned)>>` cannot be shared between threads safely
+ --> ui/protocol_object_only_protocols.rs
+  |
+  |     let _: &ProtocolObject<dyn NSObjectProtocol + Send + Sync> = ProtocolObject::from_ref(&*obj);
+  |                                                                  ------------------------ ^^^^^ `UnsafeCell<PhantomData<(*const UnsafeCell<()>, PhantomPinned)>>` cannot be shared between threads safely
+  |                                                                  |
+  |                                                                  required by a bound introduced by this call
+  |
+  = help: within `NSObject`, the trait `Sync` is not implemented for `UnsafeCell<PhantomData<(*const UnsafeCell<()>, PhantomPinned)>>`
   = help: the following other types implement trait `ImplementedBy<T>`:
             (dyn NSObjectProtocol + 'static)
+            (dyn NSObjectProtocol + Send + 'static)
+            (dyn NSObjectProtocol + Sync + 'static)
+            (dyn NSObjectProtocol + Send + Sync + 'static)
             (dyn NSAccessibilityColor + 'static)
             (dyn NSAccessibilityCustomRotorItemSearchDelegate + 'static)
             (dyn NSAccessibilityElementProtocol + 'static)
             (dyn NSAccessibilityGroup + 'static)
-            (dyn NSAccessibilityButton + 'static)
-            (dyn NSAccessibilitySwitch + 'static)
-            (dyn NSAccessibilityRadioButton + 'static)
           and $N others
+note: required because it appears within the type `objc_object`
+ --> $WORKSPACE/crates/objc-sys/src/object.rs
+  |
+  | pub struct objc_object {
+  |            ^^^^^^^^^^^
+note: required because it appears within the type `AnyObject`
+ --> $WORKSPACE/crates/objc2/src/runtime/mod.rs
+  |
+  | pub struct AnyObject(ffi::objc_object);
+  |            ^^^^^^^^^
+note: required because it appears within the type `NSObject`
+ --> $WORKSPACE/crates/objc2/src/runtime/nsobject.rs
+  |
+  | pub struct NSObject {
+  |            ^^^^^^^^
+  = note: required for `dyn NSObjectProtocol + Send + Sync` to implement `ImplementedBy<NSObject>`
 note: required by a bound in `ProtocolObject::<P>::from_ref`
  --> $WORKSPACE/crates/objc2/src/runtime/protocol_object.rs
   |
@@ -133,24 +222,52 @@ note: required by a bound in `ProtocolObject::<P>::from_ref`
   |         P: ImplementedBy<T>,
   |            ^^^^^^^^^^^^^^^^ required by this bound in `ProtocolObject::<P>::from_ref`
 
-error[E0277]: the trait bound `dyn NSObjectProtocol + Send + Sync: ImplementedBy<NSObject>` is not satisfied
+error[E0277]: `*const UnsafeCell<()>` cannot be sent between threads safely
  --> ui/protocol_object_only_protocols.rs
   |
   |     let _: &ProtocolObject<dyn NSObjectProtocol + Send + Sync> = ProtocolObject::from_ref(&*obj);
-  |                                                                  ------------------------ ^^^^^ the trait `ImplementedBy<NSObject>` is not implemented for `dyn NSObjectProtocol + Send + Sync`
+  |                                                                  ------------------------ ^^^^^ `*const UnsafeCell<()>` cannot be sent between threads safely
   |                                                                  |
   |                                                                  required by a bound introduced by this call
   |
+  = help: within `NSObject`, the trait `Send` is not implemented for `*const UnsafeCell<()>`
   = help: the following other types implement trait `ImplementedBy<T>`:
             (dyn NSObjectProtocol + 'static)
+            (dyn NSObjectProtocol + Send + 'static)
+            (dyn NSObjectProtocol + Sync + 'static)
+            (dyn NSObjectProtocol + Send + Sync + 'static)
             (dyn NSAccessibilityColor + 'static)
             (dyn NSAccessibilityCustomRotorItemSearchDelegate + 'static)
             (dyn NSAccessibilityElementProtocol + 'static)
             (dyn NSAccessibilityGroup + 'static)
-            (dyn NSAccessibilityButton + 'static)
-            (dyn NSAccessibilitySwitch + 'static)
-            (dyn NSAccessibilityRadioButton + 'static)
           and $N others
+  = note: required because it appears within the type `(*const UnsafeCell<()>, PhantomPinned)`
+note: required because it appears within the type `PhantomData<(*const UnsafeCell<()>, PhantomPinned)>`
+ --> $RUST/core/src/marker.rs
+  |
+  | pub struct PhantomData<T: ?Sized>;
+  |            ^^^^^^^^^^^
+note: required because it appears within the type `UnsafeCell<PhantomData<(*const UnsafeCell<()>, PhantomPinned)>>`
+ --> $RUST/core/src/cell.rs
+  |
+  | pub struct UnsafeCell<T: ?Sized> {
+  |            ^^^^^^^^^^
+note: required because it appears within the type `objc_object`
+ --> $WORKSPACE/crates/objc-sys/src/object.rs
+  |
+  | pub struct objc_object {
+  |            ^^^^^^^^^^^
+note: required because it appears within the type `AnyObject`
+ --> $WORKSPACE/crates/objc2/src/runtime/mod.rs
+  |
+  | pub struct AnyObject(ffi::objc_object);
+  |            ^^^^^^^^^
+note: required because it appears within the type `NSObject`
+ --> $WORKSPACE/crates/objc2/src/runtime/nsobject.rs
+  |
+  | pub struct NSObject {
+  |            ^^^^^^^^
+  = note: required for `dyn NSObjectProtocol + Send + Sync` to implement `ImplementedBy<NSObject>`
 note: required by a bound in `ProtocolObject::<P>::from_ref`
  --> $WORKSPACE/crates/objc2/src/runtime/protocol_object.rs
   |
@@ -198,13 +315,13 @@ error[E0277]: the trait bound `dyn NSCopying + Send: ImplementedBy<NSObject>` is
   |
   = help: the following other types implement trait `ImplementedBy<T>`:
             (dyn NSObjectProtocol + 'static)
+            (dyn NSObjectProtocol + Send + 'static)
+            (dyn NSObjectProtocol + Sync + 'static)
+            (dyn NSObjectProtocol + Send + Sync + 'static)
             (dyn NSAccessibilityColor + 'static)
             (dyn NSAccessibilityCustomRotorItemSearchDelegate + 'static)
             (dyn NSAccessibilityElementProtocol + 'static)
             (dyn NSAccessibilityGroup + 'static)
-            (dyn NSAccessibilityButton + 'static)
-            (dyn NSAccessibilitySwitch + 'static)
-            (dyn NSAccessibilityRadioButton + 'static)
           and $N others
 note: required by a bound in `ProtocolObject::<P>::from_ref`
  --> $WORKSPACE/crates/objc2/src/runtime/protocol_object.rs