Peter James O'Brien (8 June 1874 – 28 September 1947) was a New Zealand politician of the Labour Party.

O'Brien was born in 1874 at Forest Creek, near Castlemaine, Victoria, Australia. His parents of Irish descent were Terence O’Brien, a miner, and Bridget O’Leary. He was christened Peter James, but his first name was never used; many knew him as Jim or Briney. After having worked as a miner in various Australian states, O'Brien emigrated to New Zealand in 1904 and found employment as a coalminer in Reefton.^[1] O'Brien became active in the socialist circles that developed on the West Coast. He became president of the Westland Certificated Engine-drivers' and Firemen's Union, and was the president of the Runanga Co-operative Society.

Standing on a socialist ticket, he was voted onto the inaugural Runanga Borough Council in 1912. In 1913, he supported Paddy Webb's successful by-election campaign in the Grey electorate for the Social Democratic Party, and later became the president of the Runanga branch of the party.^[1] He was a member of the Greymouth Borough Council for eight years and he was deputy mayor for three of them. He was also a member of the Grey Power Board. O'Brien was twice president of the Grey branch of the Labour Party and helped to establish the Grey River Argus as a Labour daily newspaper.^[2]

He represented the Westland electorate in Parliament from the 1922 election, when he defeated Tom Seddon.^[3] He lost the electorate in the 1925 election to Tom Seddon, but won it back in 1928, and held it to 1947, when he died.^[3] He was the senior Government whip from 1939 until his promotion to the cabinet in 1942.^[4]

He was a Minister in the First Labour Government. He was Minister of Transport (9 December 1942 – 28 September 1947), Minister of Marine (9 December 1942 – 28 September 1947), Minister of Labour (27 June 1946 – 19 December 1946) and Minister of Mines (27 June 1946 – 19 December 1946).^[5] He remained an MP until his death, which caused a by-election held in December 1947 that was won by Labour's Jim Kent.^[6]

In 1935, he was awarded the King George V Silver Jubilee Medal.^[7]

O'Brien married Kate Teresa Flaherty on 29 June 1909 in Greymouth. He died in Wellington on 28 September 1947. His wife survived him; they had no children.^[1]