Jamaican sprinter DID NOT propose to girlfriend

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

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??

Credit: TMZ