Reason Australia, commonly referred to as the Reason Party or as simply Reason, was an Australian political party. Its leader, Fiona Patten, described the party as a "civil libertarian alternative". Patten was elected to the Victorian Legislative Council at the 2018 state election in the Northern Metropolitan Region, after formerly being elected as an Australian Sex Party member for the same seat in 2014. However, she lost re-election in 2022.

Reason was registered at the state level in Victoria and New South Wales, as well as with the Australian Electoral Commission at the federal level. The party was disbanded in March 2024. Patten subsequently joined Legalise Cannabis Australia.

In August 2017, Fiona Patten announced the launch of a new political party called Reason Australia, in part born from a merger of the Australian Sex Party and the Australian Cyclists Party. In January 2018, the Victorian Electoral Commission officially changed the party's name from "Australian Sex Party – Victoria" to "Reason Victoria".

In May 2018, the party applied to the AEC for registration for federal elections as "Reason Australia", which was approved on 30 August 2018.

In December 2019, the NSW branch of the Voluntary Euthanasia Party merged with the Reason Party, and changed its name to "Reason Party NSW".

In December 2020 it was announced that Patten would enter a coalition with independent Mildura MP, Ali Cupper who sits in the Legislative Assembly. Cupper ended the agreement on 13 May 2021.

In February 2022, Jane Caro announced that she was standing as a candidate for the party for a New South Wales Australian Senate seat in the 2022 Australian federal election. She was unsuccessful.

Patten was not re-elected at the 2022 Victorian state election, leaving Reason with no elected representatives.