Donald Thomas Brash (born 24 September 1940) is a former New Zealand politician who was Leader of the Opposition and leader of the New Zealand National Party from October 2003 to November 2006, and briefly the leader of ACT New Zealand from April to November 2011.

In 1988, Brash became Governor of the Reserve Bank of New Zealand, a position which he held for the next fourteen years. In April 2002, before the general election on 27 July, Brash resigned from his position to stand as a list MP for the National Party. He was elected, despite significant losses for National in that election. He challenged Bill English for leadership of the National Party, being elected leader on 28 October 2003. On 27 January 2004, Brash delivered his controversial Orewa Speech, expressing opposition to perceived Māori separatism, through New Zealand's equitable measures designed to benefit them.

At the 17 September 2005 general election, National under Brash's leadership made major gains and achieved what was at the time the party's best result since the institution of the mixed-member proportional electoral system in 1993 – recovering from its worst ever result in 2002. Final results placed National two seats behind the incumbent New Zealand Labour Party, with National unable to secure a majority from the minor parties to form a governing coalition.

On 27 November 2006, Brash resigned as National Party leader. He subsequently retired from Parliament in February 2007. In October 2008, he was appointed as an adjunct professor of Banking in the Business School at the Auckland University of Technology, and an adjunct professor in the School of Economics and Finance at La Trobe University in Melbourne, Australia.

On 28 April 2011, Brash was elected the party leader of ACT (a libertarian party) after his bid for its leadership was accepted and he was confirmed by the ACT caucus and board. He resigned later that year on 26 November 2011 due to ACT's poor showing in the election, and its failure to gain any seats apart from its electorate strong-hold of Epsom. In 2016, he founded the right-wing lobby group Hobson's Pledge, to seek to nullify the partnership between Māori and the Crown, and further oppose equitable measures for Māori.