Robert Adam Ross Maclennan, Baron Maclennan of Rogart, PC (26 June 1936 – 18 January 2020) was a British Liberal Democrat politician and life peer.

He was the last leader of the Social Democratic Party (SDP), serving during the negotiations that led to its merger with the Liberal Party in 1988. He then became joint interim leader of the new party, known as the Social and Liberal Democrats (SLD) and later as the Liberal Democrats. He served as a Member of Parliament (MP) from 1966 to 2001, when he was elevated to the House of Lords.

MacLennan was born in Glasgow in 1936.^[1] His father, Sir Hector MacLennan, was a renowned gynaecologist and obstetrician.^[1] His mother, Isabel Margaret (née Adam), was a physician and public health activist.^[1] He was the brother of actor and director David MacLennan, actress and writer Elizabeth MacLennan, and Kenneth MacLennan.^[1]

He was educated at Glasgow Academy; Balliol College, Oxford; Trinity College, Cambridge; and Columbia University, New York City.^[1] He was called to the bar in 1962 at Gray's Inn and before entering parliament practised as an international lawyer based in London.^[2] From 1963 to 1964, he worked at Sullivan & Cromwell in New York.^[1]

He became Member of Parliament (MP) for the constituency of Caithness and Sutherland in 1966, and serving until 1997; and for Caithness, Sutherland and Easter Ross after boundary changes, from 1997 to 2001.^[1]

He was first elected as a member of the Labour Party, where he was described as a "gentle Jenkinsite" and served as a junior minister in the Labour government of 1974–1979.^[1] In 1981 defected to become a founder member of the SDP, co-founded by Jenkins.^[1] Maclennan helped write the party's constitution.^[1] He was one of the few SDP MPs to keep their seats in the 1983 general election. After his stint as SDP Leader in 1988, he served as a front bench spokesman for the Liberal Democrats, and as their president from 1994 until 1998.^[1]

He was appointed to the Privy Council in 1997.^[3]

After his retirement at the 2001 general election, he was raised to the House of Lords, created a life peer as Baron Maclennan of Rogart, of Rogart in Sutherland.^[4][5] He was the party's Cabinet Office and Scotland spokesman in the House of Lords until 2015.

In 1968, Maclennan married Helen Noyes (née Cutter), daughter of Justice Richard Ammi Cutter of the Massachusetts Supreme Judicial Court.^[6] As Maclennan's spouse she was Lady Maclennan of Rogart. She had one son from a previous marriage, and a son and a daughter with Maclennan.^[2]

Maclennan died at his home in the City of Westminster on 18 January 2020, at age 83.^[1][2]