MJ's Bro Files Police Report -- My Asst. Is a THIEF!

Michael Jackson's brother Randy claims a no-good thief has stolen the keys to his home ... and he's pointing the finger at his very own personal assistant ... TMZ has learned.

Randy filed a police report with the Pasadena Police Department a few days ago ... claiming his former female assistant quit her job recently ... and REFUSED to give back the keys to his home.

Sources close to the ex-assistant tell TMZ ... the two ended their professional relationship on VERY bad terms ... but she's NO THIEF.

The assistant worked for Randy for roughly 6 months -- and claims she quit because she felt she was being underpaid. After the split, the woman is saying she may have already had the keys in her possession because of her duties as Jackson's assistant ... but returned them ASAP.