三国同盟（英语：Triple Alliance）是德意志帝国、奥匈帝国和意大利王国在1882年5月20日签署的条约，是一个军事联盟。三国同意任何一国被其他两个或更多国家攻击时协助对方。而且，德国与意大利约定，如果对方被法国攻击，将会协助对方。然而，根据一项补充宣言，意大利的承诺不能被认定为针对英国。1902年6月，意大利延续同盟协定后，对法国许下了类似的承诺。

1881年5月，法国进逼突尼斯这个意大利希望得到的殖民地。于是，为了得到支持，意大利加入德国和奥匈帝国的同盟，结为三国同盟。不过，意大利的民意并不太满意与奥匈帝国结盟，因为它曾阻止意大利统一，并且仍然控制着被认为是“尚未收复的意大利”（意大利语：irredentismo italiano）的特伦蒂诺和伊斯特拉。

The Triple Alliance was a secret agreement between Germany, Austria-Hungary, and Italy formed on 20 May 1882 and renewed periodically until World War I. Germany and Austria-Hungary had been closely allied since 1879. Italy sought their support against France shortly after losing North African ambitions to the French. Each member promised mutual support in the event of an attack by any other great power. The treaty provided that Germany and Austria-Hungary were to assist Italy if it were attacked by France without Italian provocation; Italy would assist Germany if Germany were attacked by France. In the event of a war between Austria-Hungary and Russia, Italy promised to remain neutral.