Usain Bolt -- My Sex Was On Fire ... At Oktoberfest! (VIDEO)

Here's video of Usain Bolt leading a tent full of drunken Germans in song at Oktoberfest this weekend in Munich ... where the Jamaican sprinter was crushing beers for the 3rd year in a row!!

Bolt LOOOVES Oktoberfest -- like super totally obsessed -- and this year, he upped his game by singing King of Leon's biggest hit "Sex On Fire" with the house band at the Schuetzen Tent.

When he wasn't singing, Bolt was spotted downing steins of brew with a whole bunch of hot chicks ... 'cause ... well ... he likes it.