Sean Patrick Astin (né Duke; born February 25, 1971) is an American actor and trade union leader who serves as the 4th national president of SAG-AFTRA. He began his career as a child actor, making his film debut as Mikey Walsh in The Goonies (1985), followed by significant roles as Billy Tepper in Toy Soldiers (1991), Dave Morgan in Encino Man (1992), Daniel Ruettiger in Rudy (1993), and Samwise Gamgee in The Lord of the Rings trilogy (2001–2003).

He is known to television audiences for portraying Lynn McGill on the fifth season of 24 (2006), the voice of Oso in Special Agent Oso (2009–2012), the voice of Raphael on Teenage Mutant Ninja Turtles (2012–17), and Bob Newby on the second season of Stranger Things (2017).

Astin's acting awards include a Screen Actors Guild Award and two Young Artist Awards. He was also nominated for the Academy Award for Best Live Action Short Film in 1994 for the short film Kangaroo Court. Astin is the son of actress Patty Duke and was adopted in 1972 by her then-husband, actor John Astin.

Astin ran for president of SAG-AFTRA in 2025 and was elected for a two-year term. His term began in September 2025.

Astin was born in Santa Monica, California, on February 25, 1971, the son of actress Patty Duke (1946–2016) and Michael Tell (1944/1945–2025). At the time, it was incorrectly reported that entertainer Desi Arnaz Jr. was his biological father. Tell was a writer, music promoter, and publisher of the newspaper The Las Vegas Israelite. When Duke became pregnant, she was unsure whether Tell, Arnaz or actor John Astin was the father, and Tell offered to marry her as a way out of the scandal. The marriage lasted only 13 days in 1970, ending before Astin was born.

In 1972, Duke married John Astin. When the wedding guests were invited to speak, 18-month-old Astin looked at John and cried, "Daddy!", to which the Episcopal priest performing the ceremony remarked, "Well, that about does it!" John subsequently adopted Sean. In 1973, Duke gave birth to Astin's half-brother Mackenzie Astin, who also became an actor. Duke and John Astin divorced in 1985. Duke married Mike Pearce in 1986, and they adopted a son, Kevin, in 1989. When Astin was 14, Duke told him that Arnaz was his biological father. Almost a decade later, in 1994, Astin met Tell's niece, who suggested that Astin get a paternity test. Tell was found to be his biological father. Astin developed close relationships with all three, saying: "Desi Arnaz Jr. loves me, and I love him." Astin considers John his father, as John raised him. Astin is also close to his stepfather, Mike Pearce, saying, "I can call any of them on the phone any time I want to. John, Desi, Mike, or Papa Mike ... my four dads."

Astin is of German and Irish ancestry through his mother, and Jewish ancestry through his biological father. Astin attended Catholic school and later became a Protestant.

Astin is a graduate of the Crossroads High School for the Arts in Santa Monica and completed master classes in acting at the Stella Adler Conservatory in Los Angeles. He attended Los Angeles Valley College before transferring UCLA, where he graduated cum laude with a bachelor of arts in history and English (American literature and culture).