Jeffrey Mark Donaldson (born 7 December 1962) is a Northern Irish politician and Member of Parliament for Lagan Valley. He is best known for his opposition to Ulster Unionist Party (UUP) leader David Trimble during the Northern Ireland peace process, especially from 1998 to 2003.

Jeffrey Donaldson was born in Kilkeel, County Down. He joined the Ulster Unionist Party as a young man and later worked for the MP Enoch Powell. In 1985 he was elected in a by-election to the Northern Ireland Assembly. In 1996 he was first placed candidate on the UUP list for the 1996 forum elections, virtually guaranteeing him a seat. This led to his selection in 1997, for the Westminster Parliament where he was elected as a Member of Parliament (MP) for the Lagan Valley (UK Parliament constituency) constituency. At that time he was widely seen as a potential future leader of the Ulster Unionist Party

Two of Donaldson's cousins were killed by the Provisional Irish Republican Army while serving in the Royal Ulster Constabulary: Sam Donaldson was killed in 1970 and Alex Donaldson, a Chief Inspector, died in a mortar attack on a Newry police station in 1985.

Donaldson stated in Richard English's book, Armed Struggle: "A big part of the motivation for me in becoming actively involved in politics was the deep sense of injustice that I felt had been perpetrated against my people and specifically against my family and I wanted to do something about that. And that's why, at the age of 18, I did two things, I joined the Ulster Defence Regiment and I joined the Ulster Unionist Party, because I wanted to pursue, through the forces of the state and the forces of law and order, the IRA and to oppose their campaign. But I also wanted to be involved politically in opposing that campaign as well". (English p. 373)

In 1998 Donaldson was in the Ulster Unionists' 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, notably the early release of political prisoners and the renaming of the Royal Ulster Constabulary to The Police Service of Northern Ireland.

Hence, the seeds were planted for dissent between the figurehead of the anti-agreement faction of the UUP (Donaldson) and the party leader (Trimble). Donaldson was not allowed to stand in the 1998 assembly election as a party rule stopped MPs, apart from the leader and deputy leader, from going forward as assembly candidates.

Donaldson engineered several party council meetings in protest against David Trimble's policies. The council, however, backed Trimble’s leadership, and in mid-2003, along with fellow MPs David Burnside and Martin Smyth, Donaldson resigned the Ulster Unionist whip at Westminster. The MPs remained party members and in November 2003 Donaldson was elected to the Northern Ireland Assembly for the UUP.

Following the success of the rival Democratic Unionist Party (DUP) in the same election Assembly election of 2003, he continued to call for Trimble's immediate resignation However the party continued to back Trimble. On 18 December 2003 Donaldson, Norah Beare MLA and Arlene Foster announced their resignation from the UUP and on January 5 2004 they announced that they had joined the Democratic Unionist Party (DUP).

Donaldson was returned to the British House of Commons in the 2005 UK general election. He is also a senior member of the loyal orders.