Nicolas Guy Barnett (23 August 1928 – 24 December 1986) was a Labour Party politician and MP in the United Kingdom parliament.
He was educated at Highgate School and St Edmund Hall, Oxford. He taught at Queen Elizabeth Grammar School, Wakefield, from 1953 to 1959 and at Friends' School, Kamusinga, in Kenya, from 1960 to 1961. He then worked for various development charities, until 1962.
After unsuccessfully contesting Scarborough and Whitby in 1959, Barnett was elected MP for South Dorset in a by-election in November 1962 after the Conservative MP Victor Montagu succeeded to the peerage as the Earl of Sandwich. Barnett only held the seat briefly as he was defeated in the 1964 general election.
He was returned to parliament as MP for Greenwich in a by-election in July 1971, upon the resignation of the Labour incumbent Richard Marsh to become chairman of British Rail, and retained the seat until his death on Christmas Eve 1986. The subsequent by-election in February 1987 was won by the SDP candidate Rosie Barnes.