Zack Snyder's Justice League has been rated R, according to Film Ratings. The film earned its rating due to "violence and some language." (H/T Fandom)
The long-awaited Snyder Cut will be making its HBO Max debut this March as one super-long film. Additional photography brought back Jared Leto's Joker – whose new look has been teased in an image by Snyder – and Joe Manganiello's Deathstroke, and the director has said about 150 minutes of the film have never been seen before. He's also already speculated that the film would score an R rating: "The movie is insane and so epic and is probably rated R – that's one thing I think will happen, that it will be an R-rated version, for sure."
Considering Steppenwolf's original, spiky design has been restored – a scarier style than the theatrical version – it’s probably fair to assume some of the R-rated violence will be coming from him. Can we expect a more violent Amazons vs Steppenwolf battle, maybe? It also seems a safe bet that we'll be seeing more from the Knightmare timeline, first glimpsed in Batman v Superman: Dawn of Justice. This is a likely candidate for action worthy of an R rating, especially as this is probably how Joker and Deathstroke are involved in the film.
