Usain Bolt -- I'M IN LOVE WITH RIHANNA ... Call Me!

Usain Bolt -- the fastest man on the planet -- is chasing HARD after Rihanna ... telling TMZ Sports he's straight up IN LOVE with the singer and wants to get together ASAP.

Bolt made his intentions clear after Rihanna posted an ass-jiggling dance video this week -- when he tweeted at the singer, "I clearly am in love frm this dancing."

He added, "If ur not Caribbean u may not understand. #sexy #seriousface #inlove #needherinmylife #now."

So when we saw the Jamaican superstar in L.A. this week -- we asked if he had a message for RiRi ... and the guy used the opportunity to officially begin his courtship.