Ex Power Ranger Ricardo Medina Jr. -- Arrested for Murder

One of the stars of "Power Rangers Samurai" has been arrested for stabbing his roommate to death with a sword ... TMZ has learned.

Ricardo Medina Jr. was busted Saturday afternoon by L.A. County Sheriff's deputies in Palmdale, CA ... after a fight broke out between himself and the roommate ... Joshua Sutter.

According to law enforcement ... Medina retreated to his bedroom with his girlfriend -- but Sutter forced his way into the bedroom ... and that's when Medina allegedly stabbed him once in the abdomen with the sword.

Medina called 911 and waited for for cops to arrive. He was arrested and booked for murder. Sutter was transported to a hospital ... where he was pronounced dead.

Medina got his break in Hollywood as the Red Ranger on "Power Rangers Wild Force" back in 2002. He went on to play Dekker on "Power Rangers Samurai."

He's still in jail, and bail has been set at one million dollars.