The Zimmerwald Conference was held in Zimmerwald, Switzerland, from September 5 to 8, 1915. It was the first of three international socialist conferences convened by anti-militarist socialist parties from countries that were originally neutral during World War I. The individuals and organizations participating in this and subsequent conferences held at Kienthal and Stockholm are known jointly as the Zimmerwald movement.

The Zimmerwald Conference began the unraveling of the coalition between revolutionary socialists (the so-called Zimmerwald Left) and reformist socialists in the Second International.