Chief Keef -- ARRESTED in Atlanta

7:18 AM PT -- Mystery solved ... it's a marijuana arrest.

According to the police report, obtained by TMZ, Keef was busted for allegedly smoking weed at a fancy hotel.

Chief Keef -- the 17-year-old rap sensation -- was arrested in DeKalb County, Georgia yesterday for disorderly conduct ... TMZ has learned.

The details surrounding the arrest are unclear.

According to the booking sheet, Keef -- real name Keith Cozart -- was listed at 5'9" and 160 lbs.

After the arrest, Keef tweeted, "Jus Got Out Of Dekalb County jail In Atlanta Mad As f**k."

Keef is considered a prodigy -- and has rapped with huge stars like 50 Cent and Wiz Khalifa.

Keef is no stranger to the law ... he's been arrested at least two times before -- and even has a gun conviction under his belt.

It's been a rough year for the rapper ... We broke the story, he was recently sued for child support​ by his middle-school-aged baby mama who claims Keef fathered her kid -- born in 2011 -- when he was only 15.