diff --git a/qiskit_ibm_runtime/api/session.py b/qiskit_ibm_runtime/api/session.py
index 54744eaa2..95420e02a 100644
--- a/qiskit_ibm_runtime/api/session.py
+++ b/qiskit_ibm_runtime/api/session.py
@@ -34,6 +34,7 @@
     503,  # Service Unavailable
     504,  # Gateway Timeout
     520,  # Cloudflare general error
+    521,  # Cloudflare web server is down
     522,  # Cloudflare connection timeout
     524,  # Cloudflare Timeout
 )