The Second International was an organization formed in 1889 (after several years of preparation) by socialist and labour parties who wished to work together for international socialism. It continued the work of the dissolved First International and was in existence until 1916.

The Second International dissolved during World War I, as the separate national parties that composed it did not maintain a unified front against the war, instead generally supporting their respective nation's role. Additionally, the Second International was not as revolutionary as the later Third and Fourth Internationals, in most countries supporting the legitimacy of electoral rule.

In 1920 the Second International was reorganized. However, some European socialist parties refused to join the reorganized international, and decided instead to form the International Working Union of Socialist Parties. In 1923 IWUSP and the Second International merged to form the Labour and Socialist International. This international continued to exist until 1940.

After World War II, the Socialist International was formed to continue the policies of the Second International, and it continues to this day.