John Archibald Sinclair, 3rd Viscount Thurso, (born 10 September 1953), known also as John Thurso, is a Scottish businessman, Liberal Democrat politician and hereditary peer.Thurso is notable for having served in the House of Lords both before and after a period in the House of Commons.
He first joined Parliament in the Lords as a hereditary peer between 1995 and 1999.
Most hereditary peers were removed from Parliament following the House of Lords Act 1999.
Subsequently, he was elected Member of Parliament (MP) for Caithness, Sutherland and Easter Ross from the 2001 election until he was defeated in the 2015 election by SNP candidate Paul Monaghan.
He was the fifth generation of the Sinclair family to represent the Caithness area in the House of Commons.
In 2016, he returned to the House of Lords after winning a by-election to fill a vacancy among the remaining Liberal Democrat hereditary peers.