Seumas Patrick Charles Milne (born 5 September 1958) is a British journalist and political aide. He was appointed as the Labour Party's Executive Director of Strategy and Communications in October 2015 under Labour Party Leader Jeremy Corbyn, initially on leave from The Guardian. In January 2017, he left The Guardian in order to work for the party full-time. He left the role upon Corbyn's departure as leader in April 2020.

Milne joined The Guardian in 1984. He was a columnist and associate editor there at the time of his Labour Party appointment, and according to Peter Popham writing for The Independent in 1997, was "on the far left of the Labour Party." Milne is the author of The Enemy Within: The Secret War Against the Miners, a book about the 1984–1985 UK miners' strike which focuses on the role of MI5 and Special Branch in the dispute.

Born in Dover, Milne is the younger son of Alasdair Milne (1930–2013), Director-General of the BBC from 1982 to 1987, and his wife Sheila Kirsten, née Graucob, who was of Irish and Danish ancestry.

Milne was educated at Tormore School, a boys' independent preparatory school in Deal, Kent, followed by Winchester College, a public school in Hampshire. In 1974, he stood in a mock election at Winchester as a Maoist Party candidate.

Following Winchester, Milne attended Balliol College, Oxford, where he read Philosophy, politics and economics, and Birkbeck, University of London, where he read Economics. While at Balliol, Milne was so committed to the Palestinian cause that he spoke with a Palestinian accent and called himself Shams (Arabic for "sun"). His sister Kirsty Milne, who died in July 2013, was an academic who had previously been a journalist.

After graduating from Oxford University, Milne became the business manager of Straight Left, a monthly publication that began in 1979, which, according to Standpoint magazine, was produced by a pro-Soviet faction in the Communist Party of Great Britain, and included several left-wing Labour MPs with pro-Soviet bloc sympathies on its editorial board. During his time at Straight Left Milne became friends with Andrew Murray, who much later again became a colleague of Milne in the Labour Party. Milne himself was not a Communist Party member.

Milne worked as a staff journalist at The Economist from 1981 but was not content working for a free-market newspaper, later describing it as "the Pravda of the neoliberal ascendancy." In 1984, he joined The Guardian on the recommendation of Andrew Knight, The Economist's then editor. Milne's early responsibilities for The Guardian included posts as news reporter, Labour Correspondent (by 1994), and Labour Editor. In 1994, Milne's colleague Richard Gott resigned from The Guardian following an article in The Spectator that alleged Gott had connections to the KGB and was a Soviet agent of influence—charges that Gott vociferously denied. Milne defended Gott against these allegations, which he thought "seemed absurd", and claimed the journalists who had written the expose of his friend were connected to MI5.

Milne was Comment Editor for six years from 2001 to 2007. According to Peter Wilby in an April 2016 New Statesman profile of Milne, his most controversial decision among The Guardian staff was to print a 2004 article by Osama bin Laden, assembled from recordings of one of his speeches. While almost all thought it should have been published, a small majority thought it should not have been run as a comment piece, although the Readers' Editor later defended this decision.