diff --git a/backend/mpc-system/services/session-coordinator/adapters/output/postgres/session_postgres_repo.go b/backend/mpc-system/services/session-coordinator/adapters/output/postgres/session_postgres_repo.go
index 323a9089..26ec866e 100644
--- a/backend/mpc-system/services/session-coordinator/adapters/output/postgres/session_postgres_repo.go
+++ b/backend/mpc-system/services/session-coordinator/adapters/output/postgres/session_postgres_repo.go
@@ -249,6 +249,15 @@ func (r *SessionPostgresRepo) Update(ctx context.Context, session *entities.MPCS
 	}
 	defer tx.Rollback()
 
+	// Lock the session row first to prevent concurrent modifications
+	// This ensures serializable isolation for the entire session update
+	_, err = tx.ExecContext(ctx, `
+		SELECT id FROM mpc_sessions WHERE id = $1 FOR UPDATE
+	`, session.ID.UUID())
+	if err != nil {
+		return err
+	}
+
 	// Update session
 	_, err = tx.ExecContext(ctx, `
 		UPDATE mpc_sessions SET
@@ -265,8 +274,16 @@ func (r *SessionPostgresRepo) Update(ctx context.Context, session *entities.MPCS
 		return err
 	}
 
-	// Update each participant individually using UPDATE to avoid lost updates
-	// Using individual UPDATE statements ensures concurrent updates to different participants don't conflict
+	// Lock all participant rows for this session to prevent concurrent modifications
+	// This prevents lost updates when multiple parties report completion simultaneously
+	_, err = tx.ExecContext(ctx, `
+		SELECT id FROM participants WHERE session_id = $1 FOR UPDATE
+	`, session.ID.UUID())
+	if err != nil {
+		return err
+	}
+
+	// Update each participant individually
 	for _, p := range session.Participants {
 		// Try UPDATE first
 		result, err := tx.ExecContext(ctx, `