Justin Jacob Long (born June 2, 1978) is an American actor, comedian, director and screenwriter. He is known for his film roles in Galaxy Quest, Jeepers Creepers, Dodgeball, Accepted, Live Free or Die Hard, He's Just Not That into You, Drag Me to Hell, and Youth in Revolt.

He voiced Alvin Seville in the live-action Alvin and the Chipmunks film series. He is also known for his personification of a Mac in Apple's "Get a Mac" advertising campaign (2006–2009), and appeared as himself in Intel's "Go PC" campaign.

During an episode of the podcast Armchair Expert with Dax Shepard, Long revealed that he was the victim of a drugging and abduction during the filming of Youth In Revolt while in Michigan.

He had a relationship with Drew Barrymore that was on and off between 2007 and 2010. He also dated Amanda Seyfried from 2013 to 2015.

In January 2022, it was reported that he was dating actress Kate Bosworth. In April 2023, the couple announced their engagement and they married in May 2023, less than two months after getting engaged.