Usain Bolt is adding a lil' bolt to his family ... 'cause the fastest man ever just announced he's expecting a baby with longtime girlfriend Kasi Bennett.
33-year-old Bolt broke the big news on Instagram ... writing, "I just want to say a KING or QUEEN is about to be HERE."
It's unclear if the the little bundle of joy is a boy or girl.
The 8x Olympic gold medalist and Bennett -- 30 years old -- have been dating for about 5 years .