"Glee" star Darren Criss hit the sidewalks of Santa Monica yesterday unannounced -- to perform for tourists on the Third Street Promenade!
Criss sang songs like "What a Wonderful World," "Since U Been Gone," "Part of Your World" and more, as the crowd kept growing around him ... surprised a TV star was just singing acoustically in a tourist hot spot.
At one point the "Glee" star got the crowd to get close and sing with him:
Darren did the appearance for Cambio's "Acoustic Corner." They'll have more video from the performance Friday.