diff --git a/Cargo.lock b/Cargo.lock
index 376fc6cbba1..3bc35e4797a 100644
--- a/Cargo.lock
+++ b/Cargo.lock
@@ -3021,7 +3021,7 @@ dependencies = [
 
 [[package]]
 name = "libp2p-gossipsub"
-version = "0.50.0"
+version = "0.51.0"
 dependencies = [
  "async-channel",
  "asynchronous-codec",
diff --git a/Cargo.toml b/Cargo.toml
index 13a64fa8461..9e219e8d718 100644
--- a/Cargo.toml
+++ b/Cargo.toml
@@ -83,7 +83,7 @@ libp2p-core = { version = "0.44.0", path = "core" }
 libp2p-dcutr = { version = "0.15.0", path = "protocols/dcutr" }
 libp2p-dns = { version = "0.45.0", path = "transports/dns" }
 libp2p-floodsub = { version = "0.48.0", path = "protocols/floodsub" }
-libp2p-gossipsub = { version = "0.50.0", path = "protocols/gossipsub" }
+libp2p-gossipsub = { version = "0.51.0", path = "protocols/gossipsub" }
 libp2p-identify = { version = "0.48.0", path = "protocols/identify" }
 libp2p-identity = { version = "0.3.0" }
 libp2p-kad = { version = "0.49.0", path = "protocols/kad" }
diff --git a/protocols/gossipsub/CHANGELOG.md b/protocols/gossipsub/CHANGELOG.md
index e8f86b6d8ae..cb9cfc8bdf7 100644
--- a/protocols/gossipsub/CHANGELOG.md
+++ b/protocols/gossipsub/CHANGELOG.md
@@ -1,3 +1,8 @@
+## 0.51.0
+- Reject messages carrying a `key` field in `ValidationMode::Anonymous`, aligning with the
+  `StrictNoSign` policy which forbids `from`, `seqno`, `signature` and `key` on the envelope.
+  See [PR 6621](https://github.com/libp2p/rust-libp2p/pull/6621).
+
 ## 0.50.0
 - Fix unbounded growth of per-peer `connected_peer.topics` from GRAFT control messages
   ([GHSA-g3g5-x568-qvqx](https://github.com/libp2p/rust-libp2p/security/advisories/GHSA-g3g5-x568-qvqx)).
diff --git a/protocols/gossipsub/Cargo.toml b/protocols/gossipsub/Cargo.toml
index aa7bb6c452e..580ae8b427a 100644
--- a/protocols/gossipsub/Cargo.toml
+++ b/protocols/gossipsub/Cargo.toml
@@ -3,7 +3,7 @@ name = "libp2p-gossipsub"
 edition.workspace = true
 rust-version = { workspace = true }
 description = "Gossipsub protocol for libp2p"
-version = "0.50.0"
+version = "0.51.0"
 authors = ["Age Manning <Age@AgeManning.com>"]
 license = "MIT"
 repository = "https://github.com/libp2p/rust-libp2p"
diff --git a/protocols/gossipsub/src/config.rs b/protocols/gossipsub/src/config.rs
index 0fd32b12c64..84a915eb372 100644
--- a/protocols/gossipsub/src/config.rs
+++ b/protocols/gossipsub/src/config.rs
@@ -43,8 +43,8 @@ pub enum ValidationMode {
     /// This setting permits messages that have no author, sequence number or signature. If any of
     /// these fields exist in the message these are validated.
     Permissive,
-    /// This setting requires the author, sequence number and signature fields of a message to be
-    /// empty. Any message that contains these fields is considered invalid.
+    /// This setting requires the author, key, sequence number and signature fields of a message to
+    /// be empty. Any message that contains these fields is considered invalid.
     Anonymous,
     /// This setting does not check the author, sequence number or signature fields of incoming
     /// messages. If these fields contain data, they are simply ignored.
diff --git a/protocols/gossipsub/src/error.rs b/protocols/gossipsub/src/error.rs
index b3c7e5ea8ea..e61301da827 100644
--- a/protocols/gossipsub/src/error.rs
+++ b/protocols/gossipsub/src/error.rs
@@ -103,14 +103,17 @@ pub enum ValidationError {
     /// The PeerId was invalid
     InvalidPeerId,
     /// Signature existed when validation has been sent to
-    /// [`crate::behaviour::MessageAuthenticity::Anonymous`].
+    /// [`crate::ValidationMode::Anonymous`].
     SignaturePresent,
     /// Sequence number existed when validation has been sent to
-    /// [`crate::behaviour::MessageAuthenticity::Anonymous`].
+    /// [`crate::ValidationMode::Anonymous`].
     SequenceNumberPresent,
     /// Message source existed when validation has been sent to
-    /// [`crate::behaviour::MessageAuthenticity::Anonymous`].
+    /// [`crate::ValidationMode::Anonymous`].
     MessageSourcePresent,
+    /// Message key existed when validation has been sent to
+    /// [`crate::ValidationMode::Anonymous`].
+    KeyPresent,
     /// The data transformation failed.
     TransformFailed,
     /// Message size was too large for topic
diff --git a/protocols/gossipsub/src/protocol.rs b/protocols/gossipsub/src/protocol.rs
index 1950abc6c8d..2053b475167 100644
--- a/protocols/gossipsub/src/protocol.rs
+++ b/protocols/gossipsub/src/protocol.rs
@@ -434,6 +434,11 @@ impl Decoder for GossipsubCodec {
                             "Message dropped. Message source was non-empty and anonymous validation mode is set"
                         );
                         invalid_kind = Some(ValidationError::MessageSourcePresent);
+                    } else if message.key.is_some() {
+                        tracing::warn!(
+                            "Message dropped. Message key was non-empty and anonymous validation mode is set"
+                        );
+                        invalid_kind = Some(ValidationError::KeyPresent);
                     }
                 }
                 ValidationMode::None => {}