John Christopher Reilly (born May 24, 1965) is an American actor. He is known as a character actor taking leading and supporting roles in independent drama films and studio comedies. He gained exposure through his supporting roles in Tony Scott's Days of Thunder (1990), Paul Thomas Anderson's films Hard Eight (1996), Boogie Nights (1997) and Magnolia (1999), and Martin Scorsese's Gangs of New York (2002) and The Aviator (2004). Reilly was nominated for the Academy Award for Best Supporting Actor and the corresponding Golden Globe Award for his role in the musical film Chicago (2002). Reilly's other notable films include The River Wild (1994), The Thin Red Line (1998), and The Hours (2002).

Reilly gained prominence for his roles in comedy films such as Cal Naughton in Talladega Nights: The Ballad of Ricky Bobby (2006). He then went on to play the title character in Walk Hard: The Dewey Cox Story (2007), and Dale Doback, one of the two step brothers, in Step Brothers (2008). He voiced the title character in the Wreck It Ralph film series (2012–⁠2018). Reilly is also known for his work in the independent films Cyrus, We Need to Talk About Kevin, and Carnage (all in 2011), The Lobster (2015), and The Sisters Brothers (2018). He combined his comedic and dramatic abilities for his portrayal of comedian Oliver Hardy in the biopic Stan and Ollie (2018) earning nominations for the Golden Globe Award and Critics' Choice Award.

Reilly is also known for his work on television. He created and starred in a Showtime comedy series Moonbase 8 (2020) and led an ensemble cast for the HBO sports drama series Winning Time: The Rise of the Lakers Dynasty (2022–2023). Reilly performs with his band John Reilly and Friends. For his role in the Broadway revival in Sam Shepard's play True West (2000) he was nominated for the Tony Award for Best Actor in a Play.