Nas' Baby Mama -- Pay Up ... Or Face Lockup!

Nas' baby mama has an ultimatum ... pay your back child support or your sorry ass is going to jail.

Carmen Bryan filed legal docs claiming Nas -- real name Nasir Jones -- owes her more than 11K in back child support for his 19-year-old daughter, Destiny.

Bryan labels Nas a "repeat criminal non supporter" and says he's intentionally screwing over his daughter by refusing to pay her college tuition and her medical bills.