Justin W. Chambers (born July 11, 1970) is an American actor and former Calvin Klein male fashion model. As of 2005, he is starring in the ABC drama series Grey's Anatomy as Dr. Alex Karev.

Chambers was born in Springfield, Ohio, the son of deputy police officers Pam and John Chambers, of German and Irish ancestry. He has a fraternal twin brother named Jason, one older brother John, Jr.(Chip); and two older sisters, Mia and Susan. He and his twin brother visited hospitals often as children due to frequent bouts with pneumonia. He attended Southeastern High School in South Charleston, Ohio. He has five children with his wife, Keisha: Isabella (b. December 1994), twins Maya and Kaila (b. June 1997), Eva (b. March 1999) and Jackson (b. 2002). Keisha is a former model agency booker whom he married in 1993. His wife and children have recently moved to Los Angeles from their home in a suburb just outside of New York City.

In late January 2008, Chambers admitted that he suffered from biological sleep disorder. The actor checked himself into Los Angeles' UCLA Medical Center to treat exhaustion due to the sleep disorder after a period of getting only two hours of sleep each week.