The Treaty of Mutual Cooperation and Security between the United States and Japan (日本国とアメリカ合衆国との間の相互協力及び安全保障条約, Nihon-koku to Amerika-gasshūkoku to no Aida no Sōgo Kyōryoku oyobi Anzen Hoshō Jōyaku), more commonly known as the U.S.-Japan Security Treaty in English and as the Anpo jōyaku (安保条約) or just Anpo (安保) in Japanese, is a treaty that permits the presence of U.S. military bases on Japanese soil, and commits the two nations to defend each other if one or the other is attacked "in the territories under the administration of Japan." Over time, it has had the effect of establishing a military alliance between the United States and Japan.

The current treaty, which took effect on June 23, 1960, revised and replaced an earlier version of the treaty, which had been signed in 1951 in conjunction with the signing of the San Francisco Peace Treaty that terminated World War II in Asia as well as the U.S.-led Occupation of Japan (1945–1952). The revision of the treaty in 1960 was a highly contentious process in Japan, and widespread opposition to its passage led to the massive Anpo protests, which were the largest popular protests in Japan's history.The 1960 treaty significantly revised the U.S.-Japan security agreement in the direction of greater mutuality between the two nations. The original 1951 treaty had contained a provision permitting the United States to use forces based in Japan throughout East Asia without prior consultation with Japan, made no explicit promise to defend Japan if Japan were attacked, and even contained a clause allowing U.S. troops to intervene in Japanese domestic disputes. These defects were remedied in the revised version of the treaty in 1960. The amended treaty included articles delineating mutual defense obligations and requiring the US, before mobilizing its forces, to inform Japan in advance. It also removed the article permitting U.S. interference in Japanese domestic affairs.The treaty also included general provisions for the further development of international understanding and improved economic collaboration between the two nations. These provisions became the basis for the establishment of the United States–Japan Conference on Cultural and Educational Interchange (CULCON), the United States–Japan Committee on Scientific Cooperation, and the Joint United States–Japan Committee on Trade and Economic Affairs, all three of which are still in operation in some form.The U.S.-Japan Security Treaty has lasted longer than any other alliance between two great powers formed after the 1648 Peace of Westphalia. The treaty had a minimum term of 10 years but provided that it would remain in force indefinitely unless one party gives one year's notice of wishing to terminate it.