diff --git a/beacon_node/beacon_chain/src/payload_envelope_verification/import.rs b/beacon_node/beacon_chain/src/payload_envelope_verification/import.rs
index 5e90667a16..7f66f77ff8 100644
--- a/beacon_node/beacon_chain/src/payload_envelope_verification/import.rs
+++ b/beacon_node/beacon_chain/src/payload_envelope_verification/import.rs
@@ -222,6 +222,14 @@ impl<T: BeaconChainTypes> BeaconChain<T> {
         block_root: Hash256,
         payload_verification_status: PayloadVerificationStatus,
     ) -> Result<Hash256, EnvelopeError> {
+        // TODO(gloas): optimistic sync is not supported for Gloas. Proto-array only tracks
+        // `payload_received` as a bool, so an optimistically-imported payload would be treated as
+        // valid with no way to invalidate it if the EL later rejects it. Reject here (covering both
+        // the gossip and range-sync paths) until fork choice can track optimistic payload status.
+        if payload_verification_status.is_optimistic() {
+            return Err(EnvelopeError::OptimisticSyncNotSupported { block_root });
+        }
+
         // Everything in this initial section is on the hot path for processing the envelope.
         // Take an upgradable read lock on fork choice so we can check if this block has already
         // been imported. We don't want to repeat work importing a block that is already imported.
diff --git a/beacon_node/beacon_chain/src/payload_envelope_verification/mod.rs b/beacon_node/beacon_chain/src/payload_envelope_verification/mod.rs
index 41f003a2f8..870969c107 100644
--- a/beacon_node/beacon_chain/src/payload_envelope_verification/mod.rs
+++ b/beacon_node/beacon_chain/src/payload_envelope_verification/mod.rs
@@ -167,6 +167,13 @@ pub enum EnvelopeError {
     EnvelopeProcessingError(EnvelopeProcessingError),
     /// Error verifying the execution payload
     ExecutionPayloadError(ExecutionPayloadError),
+    /// Optimistic sync is not supported for Gloas payload envelopes.
+    ///
+    /// Proto-array only tracks `payload_received` as a bool, so it cannot represent an
+    /// optimistically-imported payload that the EL may later invalidate. Until fork choice can
+    /// track optimistic payload status, we reject optimistic envelopes rather than treat them as
+    /// valid.
+    OptimisticSyncNotSupported { block_root: Hash256 },
     /// The envelope's beacon block was not present in fork choice at import time.
     ///
     /// Unlike [`EnvelopeError::BlockRootUnknown`] (raised during gossip verification, where the
@@ -198,6 +205,7 @@ impl EnvelopeError {
             | EnvelopeError::PriorToFinalization { .. }
             | EnvelopeError::BeaconChainError(_)
             | EnvelopeError::BeaconStateError(_)
+            | EnvelopeError::OptimisticSyncNotSupported { .. }
             | EnvelopeError::BlockRootNotInForkChoice(_)
             | EnvelopeError::InternalError(_) => false,
         }