The Two and a Half Men actor will play homeless teen Rusty on the show, according to Deadline.
The character will be introduced in the series finale of The Closer, due to air later this year.
Major Crimes will star Mary McDonnell as police captain Sharon Raydor and had been picked up for a 10-episode season.
Martin was previously a series regular on TBS sitcom The Bill Engvall Show and has also appeared in episodes of Law & Order: Criminal Intent and Love Bites.
It has been suggested that the supporting cast on The Closer - including Corey Reynolds (Detective Gabriel) Tony Denison (Flynn), G W Bailey (Provenza) and Michael Paul Chan (Tao) - could join McDonnell and Martin on Major Crimes.
"I think that all of us are interested in continuing the storytelling," said Reynolds. "We just have to figure out what way is best to launch the new show."
Major Crimes will debut on TNT in summer 2012. NBC is also developing an unrelated legal drama of the same name.
> Major Crimes star wants Kyra Sedgwick cameo