His mother is Catholic and his biological father, Michael Tell, is Jewish.
Astin married Christine Harrell on July 11, 1992. They have three daughters: Alexandra (born November 27, 1996), Elizabeth (born August 6, 2002), and Isabella (born July 22, 2005), all with the middle name Louise. His wife held the Miss Indiana Teen USA title in 1984.
Astin has a tattoo on his ankle of the word "nine" written with the Tengwar script, commemorating his Lord of the Rings involvement and his character's membership in the Fellowship of the Ring. Seven of the eight other Fellowship actors (Elijah Wood, Sean Bean, Billy Boyd, Ian McKellen, Dominic Monaghan, Viggo Mortensen and Orlando Bloom) all have the same tattoo. John Rhys-Davies' stunt double has the tattoo as well.
During the 2004 United States presidential election, Astin backed Senator John Kerry and participated in Kerry's campaign rally in Portland, Oregon, as the opening speaker. In the 2008 presidential election, Astin lent his support to Senator Hillary Clinton and made multiple campaign appearances on her behalf, including joining Clinton's daughter, Chelsea Clinton, at some stops.
Astin is a vegetarian.
He calls four people "Dad": John Astin, Desi Arnaz Jr., Mike Tell and Michael Pearce (his step-father).
Attended the same elementary school as Camilla Belle, Nicky Hilton and Paris Hilton.
Sean gained 30 pounds to secure the role of Sam on Lord of the Rings.
When Sean was 13, Steven Spielberg handpicked him to star as Mikey Walsh in The Goonies.
He is a vegetarian.