Donald Thomas Brash 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 leader of the ACT New Zealand party for seven months from April to November 2011.

In New Zealand, the leader of the Official Opposition, commonly described as the leader of the Opposition, is the politician who heads the Official Opposition. Conventionally, they are the leader of the largest political party in the House of Representatives that is not in Government. This is usually the parliamentary leader of the second-largest caucus in the House of Representatives.