Tyga -- Sued Over Malibu Mansion ... PAY UP OR GET OUT!

"Rack City" rapper Tyga owes WAY more than ten, ten, ten, twenties, and them fifties on his rented Malibu mansion -- so says Tyga's landlord ... and now, the guy's suing to get his money.

A man named Gholamreza Rezai filed the lawsuit against Tyga and his music company, claiming Tyga signed a one-year lease in June, agreeing to pay $16,000 per month to live in the luxurious pad ... but he stopped paying rent last month.

Rezai claims Tyga expressed a desire to move out by September 30th -- informing Rezai that fans kept showing up at his front door ... and Tyga was worried the home didn't offer enough security.

According to the suit, Rezai refused to terminate the lease agreement and demanded September's rent -- but Tyga still wouldn't ante up.

What's worse, Rezai claims Tyga still hadn't vacated the premises by September 30th ... and the rapper allegedly continues to live there. We called Tyga for his side of the story, but were unable to reach him.