diff --git a/redis/asyncio/cluster.py b/redis/asyncio/cluster.py
index b066a100f6..6044edc23a 100644
--- a/redis/asyncio/cluster.py
+++ b/redis/asyncio/cluster.py
@@ -1313,7 +1313,9 @@ async def initialize(self) -> None:
         startup_nodes_reachable = False
         fully_covered = False
         exception = None
-        for startup_node in self.startup_nodes.values():
+        # Convert to tuple to prevent RuntimeError if self.startup_nodes
+        # is modified during iteration
+        for startup_node in tuple(self.startup_nodes.values()):
             try:
                 # Make sure cluster mode is enabled on this node
                 try:
diff --git a/redis/cluster.py b/redis/cluster.py
index 99e5c37f9d..6e3505404a 100644
--- a/redis/cluster.py
+++ b/redis/cluster.py
@@ -1674,7 +1674,9 @@ def initialize(self):
         fully_covered = False
         kwargs = self.connection_kwargs
         exception = None
-        for startup_node in self.startup_nodes.values():
+        # Convert to tuple to prevent RuntimeError if self.startup_nodes
+        # is modified during iteration
+        for startup_node in tuple(self.startup_nodes.values()):
             try:
                 if startup_node.redis_connection:
                     r = startup_node.redis_connection