The Closer will conclude on August 13, with the first episode of spinoff series Major Crimes airing immediately afterward on TNT.
Weatherly will appear in an early season one episode as con man Arthur 'Thorn' Woodson, reports TVLine.
The charming Woodson works as an 'intuitive life strategist' for a group of Israeli mob wives, but is secretly sleeping with all of his employers.
Weatherly confirmed his casting on Twitter, writing: "It's a guest star role. Arvin Brown is directing, he's done many NCIS episodes - I play a man named Thorn. He likes the ladies. 'Nuff said."
Prior to his NCIS role, Weatherly starred in sci-fi drama Dark Angel and appeared in episodes of Ally McBeal and Charmed.
Major Crimes will star Mary McDonnell as police captain Sharon Raydor and had been picked up for a 10-episode season.
> Dallas to debut, Falling Skies to return in June on TNT
Watch a promo for TNT's summer line-up - including Major Crimes - below: