Peter Killian Gallagher (born August 19, 1955) is an American actor. Since 1980, he has played roles in numerous Hollywood films. He is best known for starring as Sandy Cohen in the television drama series The O.C. from 2003 to 2007, recurring roles such as Deputy Chief William Dodds on Law & Order: Special Victims Unit, Stacey Koons on the Showtime comedy-drama Californication, Nick on the Netflix series Grace & Frankie, and Chuck Cedar in Mr. Deeds. He also played CIA Director of Clandestine Services (DCS) Arthur Campbell on Covert Affairs.

Early life
Gallagher was born in New York City. His mother, Mary Ann (née O'Shea), was a bacteriologist, and his father, Thomas Francis Gallagher, Jr., was an advertising executive. Gallagher is the youngest of their three children. He is of Irish Catholic background and was raised in Armonk, New York. Gallagher graduated from Tufts University, where he was active in theater, appearing in such shows as Stephen Sondheim's Company and singing with the all-male a cappella group the Beelzebubs. He studied acting at the William Esper Studio.