Chris Brown -- Surprise, I'm a Father!

Chris Brown is the proud father of a 9-month-old baby girl ... TMZ has learned.

The baby's mother is a 31-year-old former model named Nia -- who we're told has known Chris for several years.

Sources connected to both Nia and Chris tell us they are on very good terms -- however they're not together romantically. We're told 25-year-old Chris is happy about being a father.

It does not appear there is a formal child support order in place -- and we do not know the nature of any informal terms of support.

One thing's for sure ... baby girl looks a lot like daddy.