The Australia Act 1986 is the short title of each of a pair of separate but related pieces of legislation: one an Act of the Commonwealth (i.e. federal) Parliament of Australia, the other an Act of the Parliament of the United Kingdom. In Australia they are referred to, respectively, as the Australia Act 1986 (Cth) and the Australia Act 1986 (UK). These nearly identical Acts were passed by the two parliaments, because of uncertainty as to whether the Commonwealth Parliament alone had the ultimate authority to do so. They were enacted using legislative powers conferred by enabling Acts passed by the parliaments of every Australian state. The Acts came into effect simultaneously, on 3 March 1986.

According to the long title of the Australian act, its purpose was "to bring constitutional arrangements affecting the Commonwealth and the States to be brought into conformity with the status of the Commonwealth of Australia as a sovereign, independent and federal nation". The Australia Act (Cth and UK) eliminated the remaining possibilities for the UK to legislate with effect in Australia, for the UK to be involved in Australian government, and for an appeal from any Australian court to a British court. This formally separated all legal ties between Australia and the United Kingdom.