Kendrick Lamar Does Free Show ... On the Back of a Moving Truck! (VIDEO)

If you like exercise and free music, Kendrick Lamar was your hero Tuesday night because he performed a concert ... from the back of a MOVING flatbed truck.

It all started with a tweet from Kendrick directing his fans to meet him in West Hollywood.

Lamar did the gig to promote his new album and as part of a marketing campaign with Reebok. He had TONS of people along for the ride as he shut down the streets and rolled through the city.

Richard Simmons would've LOVED this show!