Donald Thomas Brash (born 24 September 1940), formerly a New Zealand politician, was Leader of the Opposition, Leader of the National Party (the country's main Opposition party at that time) from 28 October 2003 to 27 November 2006, and the Leader of the ACT Party from 28 April 2011 to 26 November 2011.
Before entering Parliament, Brash was Governor of the Reserve Bank of New Zealand from 1988 to 2002.
At the New Zealand general election on 17 September 2005, 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, compared with their worst result ever in 2002 under the leadership of his predecessor, Bill English.
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.
In late November 2006 Brash resigned as National Party Leader, and then 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 30 April 2011 Brash became the Leader of ACT New Zealand 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.