Also known as Slade, the villain will be the main antagonist in the new film, which will be directed by its star, Ben Affleck.
Affleck also played Batman in the recent Batman vs Superman: Dawn of Justice.
Manganiello is best known for his role as Richie in Magic Mike and its sequel Magic Mike XXL, but has also played werewolf Aleide Herveaux in the HBO series True Blood.
The character of Deathstroke was created by Marv Wolfman and George Perez, and first appeared in a comic in 1980. He met – and defeated – Batman in 1991 as part of the ‘City of Assassins’ storyline.
Most recently, however, the character featured as a major antagonist in the hit TV show Arrow, which featured Manu Bennett as the villain. Fans have pointed out that the actor looks very like Manganiello, which could perhaps have influenced the casting choice. The similarity could also mean that the film will reference the Arrow continuity, as part of DC’s attempts to create a Cinematic Universe like Marvel’s.
This idea holds particular weights, as rumours of the character’s appearance in the upcoming film originally began circulating after Arrow’s creator Marc Guggenheim explained that Slade would not be returning to the TV series as he is “currently tied up in another DC project.”
Deathstroke is also widely rumoured to cameo in the DC answer to the Avengers, Justice League, which will hit cinemas in November of next year. The Batman film does not yet have a confirmed release date, but is rumoured to premier in 2018.