DC Entertainment president and chief creative officer Geoff Johns confirmed in an interview with The Wall Street Journal, according to The Hollywood Reporter.
The new Batman movie is directed and co-written by Affleck. It is expected to hit theaters in 2018.
In the comics, Deathstroke was introduced 1980 in "New Teen Titans No 2". The character, whose real name is Slade Wilson, is an expert assassin with super strength and stamina. On TV he has been played by Manu Bennett on CW's Arrow.
The former "True Blood" star was also among the names being considered to play the Dark Knight in "Batman v Superman: Dawn of Justice" before Affleck stepped into the cape and cowl.