Chief Keef must pay a big bill in child support.

17-year-old rap prodigy Chief Keef is making bank off his music -- GREAT NEWS -- except he knocked up a girl and now has to pay a TON in child support based off his budding income. 

TMZ broke the story, Keef -- real name Keith Cozart -- was sued for child support​ by his middle-school-aged baby mama earlier this year. She claimed Keef fathered her kid -- born in 2011 -- when he was only 15. 

According to court docs, the teenage rapper pulls in about $13,000 A MONTH!!! Pretty baller considering his age.

As a result, a judge ordered Keef to pony up $2,600 a month in child support -- based on his lucrative career -- plus $500 in day care expenses and $10,400 in previously unpaid support. A hearing is set for next month to finalize the amount. 

Congrats Keef, you've arrived.