Robert Adam Ross Maclennan, Baron Maclennan of Rogart, PC (born June 26, 1936), educated at Balliol College, Oxford, is a British Liberal Democrat politician. He was the last leader of the Social Democratic Party, serving during the negotiations that led to its 1988 merger with the Liberal Party, and became joint interim leader of the new party, then known as the Social and Liberal Democrats.

Maclennan was MP for Caithness and Sutherland (later Caithness, Sutherland and Easter Ross) in Scotland from 1966 until 2001. He was first elected as a member of the Labour Party, and served as a junior minister in the Labour government of 1974-1979, but defected to become a founder member of the SDP in 1981. After 1988 he served as a front bench spokesman for the Liberal Democrats and as their president from 1994 until 1998. He is now the party's spokesman on Europe in the House of Lords.