T-Pain to J.R. Smith -- I Know You Don't Play Football ... But Put on a Damn Helmet!

T-Pain has a very important message for New York Knicks star J.R. Smith ... always use protection.

Pain was at Supper Club in L.A. Tuesday night rolling ... out with his crew on the same type of Can-Am bike J.R. was pushing at Greystone on Sunday.

Big difference though ... J.R. was riding sans helmet, while T-Pain tells TMZ Sports he ALWAYS straps up before he rides.

T-Pain says he's ready to challenge J.R. to a race ... provided the Knicks star is willing to meet his safety requirements.

Your move, J.R.