Mark Seddon (1963- ) is a British journalist and activist in the Labour Party.

The son of a British army officer, Seddon joined the Labour Party at the age of 15 and was educated at the University of East Anglia. He worked as a political lobbyist and became editor of Tribune in 1993, a job he kept until 2004. He was elected to the National Executive Committee of the Labour Party as a Grassroots Alliance candidate in 1997 and remains an NEC member.

Seddon has repeatedly tried to find a parliamentary seat and stood in the safe Conservative seat of Buckingham in the 2001 General Election against John Bercow. In 2002, he was controversially removed from the shortlist to be Labour's candidate in the Ogmore by-election.

Seddon has been a vocal critic of the Labour government, particularly over the 2003 Invasion of Iraq. He backed Mayor of London Ken Livingstone's ultimately successful attempts to be readmitted to the Labour Party. He currently works as a freelance journalist, contributing to several newspapers, particularly The Guardian. He is writing a book, Dear Leader, a dissenter’s tale from within New Labour.