Usain Bolt's GF Did NOT Say Yes 'Cause He Didn't Ask

Usain Bolt ain't THAT fast ... he is NOT racing down the aisle to get married -- despite a photo going viral -- TMZ Sports has learned.

The pic shows Usain and Kasi Bennett in Bora Bora -- freshly lei'd -- with a Snapchat caption, "She said yes." Well, that's only half true.

Usain's agent, Ricky Simms, tells us the image is real ... Usain posted the shot of himself and Kasi, but he says the caption is a hoax created by some Internet troll. Simms says Usain did not propose to Kasi.

All we can say is, why didn't he??

Check out this video from their South Pacific vacay, and whatever you do ... wait for the OIL RUBDOWN!!

#PutaRingOnIt