Chief Keef -- Ordered to Jail ... For Blowing Off Child Support

Police in Chicago have been told to haul Chief Keef's deadbeat ass to jail ... if they see him ... after the 17-year old rapper blew off a child support hearing, TMZ has learned.

We broke the story ... Keef was ordered to appear in court on September 5 to explain why he hasn't forked over more than $20k in child support payments to his baby mama.

Problem is ... Keef no-showed -- so the judge issued an order for his arrest, which means if cops see his ass, they can drag him right to the slammer and hold him until he pays roughly 1/5th of his child support debt.