Jeffrey Mark Donaldson (born 7 December 1962) is a Northern Irish politician and Member of Parliament for Lagan Valley.

Donaldson joined the Ulster Unionist Party and served as agent for Enoch Powell, MP. In 1997 he was elected as MP for the Lagan Valley constituency and was widely seen as a potential future leader of the Ulster Unionist Party.

In 1998 Donaldson was part of the Ulster Unionist's negotiating team for the Good Friday Agreement, however he walked out of the delegation at the end of the negotiations in protest at some of the arrangements. Subsequently he was excluded from standing in 1998 for the new Northern Ireland Assembly. Donaldson became the leading anti-agreement figure in the Ulster Unionists and led several challenges to the party leader, David Trimble, all of which failed.

In November 2003 Donaldson was elected to the Northern Ireland Assembly despite being in strong disagreement with the official position of the party. Following the electoral success of the rival Democratic Unionist Party in the Northern Ireland Assembly Election, 2003, in which the DUP won more seats than the UUP, Donaldson called for Trimble's immediate resignation. When the party failed to move against Trimble and instead backed him, on 18 December 2003 Donaldson and two other Ulster Unionist Assembly members announced that they were resigning from the party. On January 5 2004 they announced they were joining the Democratic Unionist Party.