The Marriage Amendment Bill receives royal assent and comes into effect, making Australia the 26th country to legalize same-sex marriage.

The Marriage Amendment (Definition and Religious Freedoms) Act 2017 (Cth) is an Act of the Parliament of Australia, which legalises same-sex marriage in Australia by amending the Marriage Act 1961 to allow marriage between two persons of marriageable age, regardless of their gender.

Australia, officially the Commonwealth of Australia, is a country comprising the mainland of the Australian continent, the island of Tasmania and numerous smaller islands. It has a total area of 7,688,287 km² (2,968,464 sq mi), making it the sixth-largest country in the world and the largest in Oceania. Australia is the world's flattest and driest inhabited continent. It is a megadiverse country, and its size gives it a wide variety of landscapes and climates including deserts in the interior and tropical rainforests along the coast.

Same-sex marriage, also known as gay marriage, is the marriage of two people of the same legal sex. As of 2025, marriage between same-sex couples is legally performed and recognized in 38 countries, with a total population of 1.5 billion people. The most recent jurisdiction to legalize same-sex marriage is Thailand.