"Captain" James Maclaine (occasionally "Maclean", "MacLean", or "Maclane") (1724 – 3 October 1750) was an Irish man of a respectable presbyterian family who had a brief but notorious career as a mounted highwayman in London with his accomplice William Plunkett. He was known as "The Gentleman Highwayman" as a result of his courteous behaviour during his robberies, and obtained a certain kind of celebrity. Notoriously, he held up and robbed Horace Walpole at gunpoint: eventually he was hanged at Tyburn.

The film Plunkett & Macleane was based loosely on his exploits.

Maclaine was the younger of two sons of a Scots-Irish presbyterian minister, the Revd. Thomas (?or Lauchlin) Maclaine of 1st Monaghan Presbyterian Church in Ireland. His mother, Elizabeth (née Milling) died when he was five or six years old, and his father when he was sixteen or seventeen. He came of a family of many ministers, his grandfather (a Gaelic-speaking clergyman in the Church of Scotland) having received a calling to Ireland from Argyllshire in 1698. His elder brother Archibald Maclaine (1722–1804) was educated in Glasgow and followed his own vocation as presbyterian minister, scholar and royal preceptor in the Netherlands between 1746 and 1796, famous as the first translator (1765) of Johann Lorenz von Mosheim's Ecclesiastical History (of 1726).

Educated to become a merchant, James Maclaine frittered away his inheritance in Dublin on fine clothes, gambling and prostitutes. Such inheritance as James received had mostly been dissipated by the time he was 20, and his mother's relations soon quarreled with him and refused to give him any assistance. He attached himself to the domestic service of a Mr Howard in order to travel to England, and stayed with him for a while, but got into low company and, leaving without testimonials, returned to Ireland. His mother's kin still refusing to help, his brother sent him subsidies and letters of good advice from The Hague: these proving insufficient, he considered joining the Irish Brigade in the French army, but was told that he would make little progress with them unless he became a Roman Catholic, which he was unwilling to do.

He next persuaded his old master, Colonel T-----n, to take him to England as a domestic servant, planning to acquire his fortune through an advantageous marriage. He borrowed money from the Colonel to purchase a commission, but that went the same way, and he considered enlisting in Lord Albemarle's horse troops. He disgraced himself by having an affair with an officer's wife. His friends in London raised a small subscription to enable him to ship for Jamaica, but he took the money to the Masquerades (i.e. the public gardens) and squandered it at the gaming tables. At length he found himself a wife, the daughter of an innkeeper or horse-dealer in Oxford Road, London, whom he married in about 1746.

With the dowry of five hundred pounds Maclaine set himself up as a grocer in Welbeck Street, Cavendish Square. This was no great success, however, as he did not apply himself very much, though his neighbours could not afterwards point to any real misdemeanour in his behaviour at that time. Two daughters were born to them, one of whom survived, but his wife died within three years. His mother-in-law, who had a good opinion of Maclaine, took charge of the surviving daughter. It was during his wife's illness that he met William Plunkett, an apothecary, who attended her. His business failing, owing (by his own account) to "an unavoidable trust reposed in servants", he sold off his stock and with whatever remained he turned his mind again to a military career.

However, in passing himself off as a young gentleman in search of another wealthy wife, he had neither the funds nor the habitude necessary to rise above the accompanying inducements to profligacy. Plunkett is said to have acted the part of gentleman's assistant as Maclaine, immaculately presented, paid court to a young lady worth more than £10,000 a year; but on one occasion when they had followed her to a spa, Maclaine had an argument with an apothecary in the public rooms. This person employed a military man to kick Maclaine down the stairs, while stating publicly that he knew Maclaine had been a footman only a few years previously. Plunkett and Maclaine were obliged to make a hasty and informal departure.

By his own account, Maclaine embarked on his career of crime about six months after his wife's death. At his interrogation he was encouraged to lay evidence against his accomplices, and at his trial he sought to lay a good deal of the blame for his course of life upon Plunkett (who was never caught or tried), so his narrative may not be entirely accurate. He claimed that Plunkett, a fellow-Irishman, had led him to believe he had travelled abroad, and persuaded Maclaine to employ him in his household and to lend him £100, part of which was paid back sporadically. Maclaine explained his melancholy to Plunkett as his money was running out, and Plunkett replied,