Usain Bolt Announces He's Having A Baby W/ GF Kasi Bennett!!

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."

Kasi added ... "Our golden child ❤️ Coming soon..."

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 ... and haven't been shy about their desire to have kids.

'We just got really serious, we are taking it one step at a time. I definitely want a family, though. For me, over the years, I've waited because I want to make sure it's the right person," ... Bolt told People Magazine in 2016.

"I’ve always said I want three kids. The other day I hung out with my buddy who has three kids and it was crazy. I was like, 'Hmm do I really want 3 kids?' But yes, yes I do. I’m sure of it."