Casey Affleck (born Caleb Casey McGuire Affleck-Boldt; August 12, 1975) is an American actor. He is the recipient of various accolades, including an Academy Award, a British Academy Film Award, and a Golden Globe Award. The younger brother of actor Ben Affleck, he began his career as a child actor, appearing in the PBS television film Lemon Sky (1988). He later appeared in three Gus Van Sant films: To Die For (1995), Good Will Hunting (1997), Gerry (2002), and in Steven Soderbergh's Ocean's film series (2001–2007). His first leading role was in Steve Buscemi's independent comedy-drama Lonesome Jim (2006).

Affleck's breakthrough came in 2007, when he was nominated for the Academy Award for Best Supporting Actor for his performance as Robert Ford in the Western drama The Assassination of Jesse James by the Coward Robert Ford and starred in his brother's crime drama Gone Baby Gone. In 2010, he directed the mockumentary I'm Still Here. He went on to appear in Tower Heist (2011), ParaNorman (2012), and Interstellar (2014), and he received praise for his performance as an outlaw in Ain't Them Bodies Saints (2013).

In 2016, Affleck starred in the drama Manchester by the Sea, in which his performance as a grieving man earned him the Academy Award for Best Actor. He has since starred in the dramas A Ghost Story (2017) and The Old Man & the Gun (2018), and as Boris Pash in the biographical thriller Oppenheimer (2023), his highest-grossing release.

Caleb Casey McGuire Affleck-Boldt was born on August 12, 1975, in Falmouth, Massachusetts, to Christine Anne "Chris" Boldt and Timothy Byers Affleck. The surname "Affleck" is of Scottish origin. He also has Irish, German, English, and Swedish ancestry.^{[full citation needed]} Affleck's maternal great-great-grandfather, Heinrich Boldt, known for the discovery of the Curmsun Disc, emigrated from Prussia in the late 1840s. His mother was a Radcliffe College- and Harvard-educated elementary school teacher. His father worked sporadically as an auto mechanic, a carpenter, a bookie, an electrician, a bartender, and a janitor at Harvard University. In the mid-1960s, he had been a stage manager, director, writer and actor with the Theater Company of Boston. During Affleck's childhood, his father was "a disaster of a drinker". Affleck first started acting by "reenacting what was happening at home" during role play exercises at Alateen meetings.

After his parents divorced when he was nine, Affleck and his older brother, Ben, lived with their mother and visited their father weekly. He learned to speak Spanish during a year spent traveling around Mexico with his mother and brother when he was ten. At the time, his brother Ben was filming The Second Voyage of the Mimi, which was set in Mexico. The two brothers spent "all of our time together, pretty much. Obviously at school we were in different grades, but we had the same friends." When Affleck was fourteen, his father moved to Indio, California, to enter a rehabilitation facility, and later worked there as an addiction counselor. He reconnected with his father during visits to California as a teenager: "I got to know him, really, because he was sober for the first time ... The man I knew before that was just completely different."

Growing up in a politically active, liberal household in Central Square, Cambridge, Affleck and his brother were surrounded by people who worked in the arts, were regularly taken to the theater by their mother, and were encouraged to make their own home movies. The brothers sometimes appeared in local weather commercials and as film extras because of their mother's friendship with a local casting director. Affleck acted in numerous high school theater productions while a student at Cambridge Rindge and Latin School. He has said he "wouldn't be an actor" if not for his high school theater teacher Gerry Speca: "He kind of turned me on to acting, why it can be fun, how it can be rewarding."

At age eighteen, Affleck moved to Los Angeles for a year to pursue an acting career, and lived with his brother and their childhood friend Matt Damon. Despite having "the best possible first experience" while filming To Die For, he spent much of the year working as a busboy at a restaurant in Pasadena. He decided to move to Washington, D.C., to study politics at George Washington University. He soon transferred to Columbia University in New York City, where he followed the Core Curriculum for a total of two years. However, he did not graduate: "I would do a semester of school, go do a movie ... Opportunities kept presenting themselves that were hard for me to turn down ... By then, I didn't really have roots at the school or a group of friends."

Affleck acted professionally during his childhood due to his mother's friendship with a Cambridge-area casting director, Patty Collinge. In addition to local weather commercials and film extra work, he appeared as Kevin Bacon's brother in the PBS television film Lemon Sky (1988), directed by Collinge's husband Jan Egleson, and as a young Robert F. Kennedy in the ABC miniseries The Kennedys of Massachusetts (1990). These early acting experiences "meant nothing more than a day off from school" to Affleck, and he only began to consider a career as an actor when in high school. When he later moved to Los Angeles to pursue an acting career in earnest, his first film role was as a sociopathic teenager in Gus Van Sant's 1995 satirical comedy To Die For. During filming in Toronto, Affleck shared an apartment with co-star Joaquin Phoenix and they became close friends. Peter Travers of Rolling Stone praised Affleck's performance, saying he "skillfully capture[s] the pang of adolescence among no-hopers." However, Affleck then had a "disappointing" experience while making the 1996 drama Race the Sun and, "as soon as the film finished, I went to school."