John Stamos -- I'm Back With the Beach Boys!

John Stamos reunited with the Beach Boys and brought the house down.

Stamos -- who has been a regular at shows in the past -- performed at the L.A. County fair for the first time since his post DUI rehab stint in July ... and he looked better than ever.

Stamos has been killing the game recently -- with both "Fuller House" and "Grandfathered" in production.

The Beach Boys welcomed him back as images of Uncle Jesse flashed on the big screens ... right before he lit up the crowd with a sick drum solo.

Have mercy.