The Michael Rubin staffer arrested at his Hollywood Hills mansion is one of his housekeepers, and she'd allegedly been skimming tons of cash out of his safe ... TMZ Sports has learned.

Law enforcement sources tell us the woman was taking the money out of a safe at the home of the Fanatics CEO for nearly a year. At some point, Rubin noticed cash was going missing, and we're told his security team installed a hidden camera inside the safe.

Our sources say she dipped her hand in the cookie jar again Wednesday morning -- allegedly grabbing $20,000 -- and once they had her on camera, someone called LAPD. Officers responded and immediately arrested her on the spot for grand theft.

All told, she allegedly stole $130,000.

As for how the housekeeper repeatedly got access to the safe -- we're told it's a combination safe with a backup key entry, and she allegedly got hold of the key.

Michael was not home at the time of her arrest, but he was still in L.A. after hosting several NBA All-Star weekend events.