350.org is an international environmental organization. It was started by author Bill McKibben, with the goal of building a global grassroots movement to fight against global warming and the climate change denial (en) of global warming.

350.org works with over 300 other organizations around the world. It has had people supporting the organization as "messengers". Among them are Archbishop Desmond Tutu, Alex Steffen, Bianca Jagger and David Suzuki. Van Jones and Gus Speth are on 350's U.S. Advisory Council. 350.org has a Board of Directors .

The 350s decade ran from January 1, 350, to December 31, 359. January 18 – Western Roman Emperor Constans I makes himself extremely unpopular; one of his generals, Magnentius, is proclaimed emperor at Augustodunum in the Diocese of Galliae, with the support of the army on the Rhine frontier.