Sir John James Duncan (12 February 1845 – 8 October 1913) was a politician in the colony and State of South Australia.
Duncan was born the elder son of (sea) Captain J. Duncan (died 24 April 1880) in Anstruther, Fifeshire, Scotland, and came out to South Australia with his parents in 1854; his father was a partner with his brother-in-law Sir Walter W. Hughes, who was running sheep and cattle at Hoyle's Plains and on Yorke Peninsula in the vicinity of Wallaroo and Moonta.
He was first educated privately, then at Bentley (near Gawler), Stanley Grammar School at Watervale, then at St. Peter's College. On leaving school he found employment as a clerk for Elder, Smith, & Co., then was put in charge of the finance department of the smelting works, and then the mines at Wallaroo. He then took charge of several pastoral properties of his uncle, on whose death he inherited the Gum Creek (near Burra) and Hughes Park estates. The latter property had an ideal country house to the west of Watervale, where he spent the summer months, and concentrated on sheep-breeding.
In 1871, at 26 years of age, he was elected to the Port Adelaide seat in the South Australian House of Assembly, then when that district was broken up in 1875, to the seat of Wallaroo. He resigned in 1877 to make a trip to Europe, and while in France acted as a commissioner for South Australia at the Paris Exhibition in 1878. In 1884 he was elected to the seat of Wooroora, and held that seat until 1890 when he resigned to assist in creating the National Defence League. The following year he was elected to the Legislative Council for the North-Eastern district. In 1896 he retired to visit England. In 1900 he was returned unopposed, and from 1901 was leader of the Liberal Party, and served on the Legislative Council until his death. He was gracious in manners and was deeply respected by his political opponents.