- Joaquin Phoenix as Arthur Fleck / Joker: A mentally ill, impoverished party clown and stand-up comedian disregarded by society,[6] whose history of abuse causes him to become a nihilistic criminal with a clown-inspired persona.[7] Phoenix had been interested in a low-budget "character study" of a comic book character and said the film "feels unique, it is its own world in some ways and maybe [...] It might as well be the thing that scares you the most".[8] Phoenix lost 52 pounds (23 kg) for his role in the film.[9]
- Robert De Niro as Murray Franklin:[10] A talk show host who plays a role in Arthur's downfall.[11] De Niro said his role in Joker pays homage to his character from The King of Comedy (1982), Rupert Pupkin, who is a comedian obsessed with a talk-show host.[10]
- Zazie Beetz as Sophie Dumond:[12] A cynical single mother and Arthur's "love interest".[12][13] Beetz, a huge fan of Phoenix, said that it was an honor to costar with him[14] and that she learned a lot working with him on set.[15]
- Frances Conroy as Penny Fleck: Arthur's mentally and physically ill mother,[16] who formerly worked for Thomas Wayne.[17]
- Hannah Gross as young Penny.[18]
Additionally, Brett Cullen portrays Thomas Wayne, a billionaire running for mayor of Gotham.[19] Alec Baldwin was initially cast in the role, but dropped out due to scheduling conflicts.[20][21] Carrie Louise Putrello portrays Thomas's wife Martha, although the character has no lines and is never referred to by name.[22] Douglas Hodge plays Alfred Pennyworth, the butler and caretaker of the Wayne family,[23] and Dante Pereira-Olson plays Bruce Wayne, Thomas' son, who becomes the Joker's archenemy Batman as an adult.[24][25]
Additional cast members include Glenn Fleshler and Leigh Gill as Randall and Gary, Arthur's clown co-workers;[26][27] Bill Camp and Shea Whigham as detectives Garrity and Burke in the Gotham City Police Department;[28] Marc Maron as Gene Ufland, a producer on Franklin's show;[29][30] Sharon Washington as Arthur's social worker; Josh Pais as Hoyt Vaughn, Arthur's agent;[26][31] Brian Tyree Henry as Clark, a clerk at Arkham State Hospital;[32] Ben Warheit, Michael Benz and Carl Lundstedt as bankers who harass Arthur;[33] Gary Gulman and Sam Morril as comedians at an open mic where Arthur performs;[33][34] and Bryan Callen as Javier, a co-worker of Arthur.[35] Justin Theroux has an uncredited cameo as Ethan Chase, a celebrity guest on Franklin's show.[36]
