diff --git a/src-opam/distro.ml b/src-opam/distro.ml
index d0c44fc..d21b5c9 100644
--- a/src-opam/distro.ml
+++ b/src-opam/distro.ml
@@ -756,6 +756,7 @@ let distro_arches ov (d : t) =
   | `Windows (`Msvc, _), ov when OV.major ov >= 5 -> []
   (* 2021-04-19: should be 4.03 but there's a linking failure until 4.06. *)
   | `Windows (`Msvc, _), ov when OV.(compare Releases.v4_06_0 ov) = 1 -> []
+  | `Cygwin _, ov when OV.(compare Releases.v5_1_0 ov) = 1 -> []
   | _ -> [ `X86_64 ]
 
 let distro_supported_on a ov (d : t) = List.mem a (distro_arches ov d)
@@ -868,7 +869,7 @@ let builtin_ocaml_of_distro (d : t) : string option =
   | `OracleLinux `V7 -> Some "4.01.0"
   | `OracleLinux `V8 -> Some "4.07.0"
   | `OracleLinux `V9 -> Some "4.11.1"
-  | `Cygwin _ -> None
+  | `Cygwin _ -> Some "4.14.0"
   | `Windows _ -> None
   | `Debian (`Testing | `Unstable) -> assert false
 
@@ -1257,7 +1258,7 @@ type package_manager =
   [ `Apt | `Yum | `Apk | `Zypper | `Pacman | `Cygwin | `Windows ]
 [@@deriving sexp]
 
-let package_manager (t : t) =
+let package_manager (t : t) : package_manager =
   match t with
   | `Ubuntu _ -> `Apt
   | `Debian _ -> `Apt
diff --git a/src-opam/opam.ml b/src-opam/opam.ml
index 2ee3f71..fb952ea 100644
--- a/src-opam/opam.ml
+++ b/src-opam/opam.ml
@@ -427,6 +427,22 @@ let pacman_opam2 ?(labels = []) ?arch ~opam_hashes distro () =
   @@ Linux.Pacman.add_user ~uid:1000 ~sudo:true "opam"
   @@ install_bubblewrap_wrappers @@ Linux.Git.init ()
 
+(* Cygwin based Dockerfile *)
+let cygwin_opam2 ?win10_revision ?(labels = []) ?arch ~opam_hashes distro () =
+  let opam_master_hash, opam_branches = create_opam_branches opam_hashes in
+  parser_directive (`Escape '`')
+  @@ comment "Autogenerated by OCaml-Dockerfile scripts"
+  @@ header ?win10_revision ?arch distro
+  @@ label (("distro_style", "cygwin") :: labels)
+  @@ user "ContainerAdministrator"
+  @@ Windows.sanitize_reg_path ()
+  @@ Windows.Cygwin.(install_cygwin ~extra:(cygwin_packages ()) ())
+  @@ Windows.Cygwin.Git.init ()
+  @@ install_opams opam_master_hash opam_branches
+  @@ from ?arch distro
+  @@ copy_opams ~src:"/usr/local/bin" ~dst:"/usr/bin" opam_branches
+  @@ Linux.Git.init ()
+
 let install_winget ?win10_revision ?winget version =
   match winget with
   | None when Windows.Winget.is_supported version ->
@@ -550,9 +566,7 @@ let gen_opam2_distro ?win10_revision ?winget ?(clone_opam_repo = true) ?arch
         | `Windows (`Msvc, _) ->
             windows_msvc_opam2 ?win10_revision ?winget ?labels ~opam_hashes d ()
         | _ -> assert false)
-    | `Cygwin ->
-        failwith
-          "OCaml/opam Docker images with the Cygwin port are not supported."
+    | `Cygwin -> cygwin_opam2 ?win10_revision ?labels ?arch ~opam_hashes d ()
   in
   let clone =
     if clone_opam_repo then
@@ -579,8 +593,12 @@ let ocaml_depexts distro v =
   | `Yum -> as_root (RPM.install "%s") (RPM.ocaml_depexts v)
   | `Zypper -> as_root (Zypper.install "%s") (Zypper.ocaml_depexts v)
   | `Pacman -> as_root (Pacman.install "%s") (Pacman.ocaml_depexts v)
-  | `Windows -> empty
-  | `Cygwin -> empty
+  | `Windows -> (
+      match distro with
+      | `Windows (`Mingw, _) -> Windows.Cygwin.(install (mingw_depexts v))
+      | `Windows (`Msvc, _) -> empty
+      | _ -> assert false)
+  | `Cygwin -> Windows.Cygwin.(install (cygwin_depexts v))
 
 let create_switch ~arch distro t =
   let create_switch switch pkg =
diff --git a/src-opam/windows.ml b/src-opam/windows.ml
index 97eca39..38f4ed3 100644
--- a/src-opam/windows.ml
+++ b/src-opam/windows.ml
@@ -234,10 +234,12 @@ module Cygwin = struct
          (if upgrade then " --upgrade-also" else ""))
       fmt
 
-  let install ?(cyg = default) pkgs =
-    cygsetup ~cyg "--packages %s"
-      (pkgs |> List.sort_uniq String.compare |> String.concat ",")
-    |> cleanup
+  let install ?(cyg = default) = function
+    | [] -> empty
+    | pkgs ->
+        cygsetup ~cyg "--packages %s"
+          (pkgs |> List.sort_uniq String.compare |> String.concat ",")
+        |> cleanup
 
   let update ?(cyg = default) () = cygsetup ~cyg ~upgrade:true "" |> cleanup
 
@@ -252,7 +254,7 @@ module Cygwin = struct
          ~dst:(cyg.root ^ {|\setup-x86_64.exe|})
          ()
     @@ install_cygsympathy_from_source cyg
-    @@ (if extra <> [] then install ~cyg extra else empty)
+    @@ install ~cyg extra
     @@ (if msvs_tools then install_msvs_tools_from_source cyg else empty)
     @@ run
          {|awk -i inplace "/(^#)|(^$)/{print;next}{$4=""noacl,""$4; print}" %s\etc\fstab|}
@@ -266,8 +268,7 @@ module Cygwin = struct
       from
     @@ workdir {|%s\home\opam|} cyg.root
 
-  let cygwin_packages ?(flexdll_version = "0.39-1") () =
-    (* 2021-03-19: flexdll 0.39 is required, but is in Cygwin testing *)
+  let cygwin_packages ?flexdll_version () =
     [
       "make";
       "diffutils";
@@ -277,7 +278,9 @@ module Cygwin = struct
       "patch";
       "m4";
       "cygport";
-      "flexdll=" ^ flexdll_version;
+      (match flexdll_version with
+      | Some v -> "flexdll=" ^ v
+      | None -> "flexdll");
     ]
 
   let mingw_packages =
@@ -285,6 +288,16 @@ module Cygwin = struct
 
   let msvc_packages = [ "make"; "diffutils"; "patch"; "git" ]
 
+  let cygwin_depexts v =
+    if Ocaml_version.compare v Ocaml_version.Releases.v5_1_0 >= 0 then
+      [ "libzstd-devel" ]
+    else []
+
+  let mingw_depexts v =
+    if Ocaml_version.compare v Ocaml_version.Releases.v5_1_0 >= 0 then
+      [ "mingw64-x86_64-zstd" ]
+    else []
+
   let install_ocaml_for_windows ?cyg ?(version = "0.0.0.2") () =
     let packages =
       [
diff --git a/src-opam/windows.mli b/src-opam/windows.mli
index fa86a33..7aa24e3 100644
--- a/src-opam/windows.mli
+++ b/src-opam/windows.mli
@@ -108,8 +108,7 @@ module Cygwin : sig
   (** Setup Cygwin workdir, optionally copied from the [from] Docker image. *)
 
   val install : ?cyg:cyg -> string list -> Dockerfile.t
-  (** Install the supplied Cygwin package list. The packages should be
-      comma-separated. *)
+  (** Install the supplied Cygwin package list. *)
 
   val update : ?cyg:cyg -> unit -> t
   (** Update Cygwin packages. *)
@@ -120,11 +119,19 @@ module Cygwin : sig
 
   val mingw_packages : string list
   (** [mingw_packages] is the list of base development tools for the
-      Caml mingw port. *)
+      OCaml mingw port. *)
 
   val msvc_packages : string list
   (** [msvc_packages] is the list of base development tools for the
-      Caml MSVC port. *)
+      OCaml MSVC port. *)
+
+  val cygwin_depexts : Ocaml_version.t -> string list
+  (** [cygwin_depexts v] returns packages that are required by the
+      OCaml Cygwin distribution at version [v]. *)
+
+  val mingw_depexts : Ocaml_version.t -> string list
+  (** [mingw_depexts v] returns packages that are required by the
+      OCaml mingw distribution at version [v]. *)
 
   val install_ocaml_for_windows :
     ?cyg:cyg -> ?version:string -> unit -> string list * t