If you need a reminder that Lakers coach Luke Walton was once one of the best athletes on the planet ... we have video of him KILLING it at beach volleyball.

The video was posted by pro volleyball players the McKibbin Brothers, and shows Walton holding his own on a court with AVP pros Casey Patterson, Sean Rosenthal, Stafford Slick and Chase Budinger.

Walton is blockin', spikin', and settin' like one of the pros ... using his 6'8" frame to dominate out there on the sand, and we're being serious ... THE DUDE CAN REALLY PLAY.