Captain John Porteous (c. 1695 – 7 September 1736) was a Scottish military officer who served in the Edinburgh City Guard and played a major role in the Porteous Riots, which resulted in his death.

John Porteous was born at The Glen, Quair Water, near Traquair, in the Borders, the son of Stephen Porteous, a tailor of the Canongate, Edinburgh. Little is known of his early life, except that he soon found he was not suited to his father's calling. Having served in the Scots Brigade in the Dutch Republic, in 1716 he became drill master to the Edinburgh force in anticipation of a Jacobite rising, and two years later was appointed as ensign to the City Guard, and was promoted in 1726 to the rank of captain. Porteous seems to have been an overbearing official, despised by the mob and the underclasses of Edinburgh society.

In 1724 he lost a wager of 20 guineas in playing a game of golf on Leith Links against Alexander Elphinstone, the brother of Lord Balmerino. The wager attracted a small crowd, including the Duke of Hamilton and George Douglas.

On 14 April 1736, three convicted smugglers, Andrew Wilson, William Hall and George Robertson, were arrested, tried and condemned to death. Hall's sentence was commuted to transportation for life, while Wilson and Robertson awaited their fate. A few days before the execution George Robertson was enabled by Wilson to escape from his guards while attending a service in St Giles', shouting "Run Geordie, run" as he held two officers and clamped his teeth onto the collar of a third. With the help of sympathetic supporters George Robertson eventually made his way to the Dutch Republic.

The remaining convict, Andrew Wilson, was taken to be publicly hanged in the Grassmarket, Edinburgh, on 14 April 1736. His body was cut down against the wishes of the mob, and the ensuing riot was such that the hangman had to be placed in protective custody. As the situation worsened, for fear of an attempt to rescue the victims, the Lord Provost of Edinburgh instructed Porteous to call out the entire guard and to furnish them with powder and shot.

After the execution, the mob became violent and began to stone the City Guard. Accounts of events are confused, but what is certain is that Porteous instructed his men to fire above the heads of the crowd, but in so doing, they wounded people in the windows of the high tenement buildings opposite. The crowd became increasingly violent and, as panic set in, Captain Porteous ordered the guard to shoot into the mob, which led to the deaths of six people in all.

Porteous was arrested the same afternoon and charged with murder. He was tried at the High Court of Justiciary on 5 July 1736, where a majority of witnesses testified that Porteous had personally fired into the crowd on 14 April, although sixteen others said they had not seen him do so. Feelings were running high in Edinburgh and the jury unanimously found Porteous guilty of murder. He was sentenced to death, the execution was set to take place in the Grassmarket on 8 September 1736. Porteous was imprisoned in the Tolbooth prison, near St Giles church.

Events in Scotland alarmed government officials in London, and Sir Robert Walpole attempted to influence events by asking his representative in Edinburgh to become involved. But he had miscalculated, underestimating the depth of feeling in Scotland. A formal appeal was petitioned and the execution was deferred.