The Australia, New Zealand, United States Security Treaty (ANZUS or ANZUS Treaty) is a 1951 non-binding collective security agreement initially formed as a trilateral agreement between Australia, New Zealand, and the United States; and from 1986 an agreement between New Zealand and Australia, and separately, Australia and the United States, to co-operate on military matters in the Pacific Ocean region, although today the treaty is taken to relate to conflicts worldwide. It provides that an armed attack on any of the three parties would be dangerous to the others, and that each should act to meet the common threat. It set up a committee of foreign ministers that can meet for consultation.

The treaty was one of the series that the United States formed in the 1949–1955 era as part of its collective response to the threat of communism during the Cold War. New Zealand was suspended from ANZUS in 1986 as it initiated a nuclear-free zone in its territorial waters; in late 2012, New Zealand lifted a ban on visits by United States warships leading to a thawing in tensions. New Zealand maintains a nuclear-free zone as part of its foreign policy and is partially suspended from ANZUS, as the United States maintains an ambiguous policy whether or not the warships carry nuclear weapons and operates numerous nuclear-powered aircraft carriers and submarines; however New Zealand resumed key areas of the ANZUS treaty in 2007.

ANZUS was overshadowed in late 2021 by AUKUS, a trilateral security pact between Australia, the United Kingdom, and the United States. It involves cooperation in nuclear submarines that New Zealand will not support. Australia and New Zealand, "are poles apart in terms of the way they see the world....I think this alliance underlines that they’re going in very different directions,” said Geoffrey Miller, an international analyst at the Democracy Project in New Zealand.