Seddon, who has quit to concentrate on his attempt to be re-elected to the Labour Party's national executive committee, has been at the title's helm during one of the most dramatic periods in its 67-year history.
Just last year, the title faced financial ruin and even stopped paying contributors, but was salvaged thanks to a £300,000 rescue package put together by trade unions.
Seddon also oversaw its evolution from a weekly newspaper to a glossy magazine in April 2001 and the title's move to new premises at the union Aslef's headquarters in Hampstead, London. The title had previously been based at the TGWU's offices on Gray's Inn Road.
Although stepping down from the top role, Seddon has pledged to continue working for the title, taking on the role of political editor. Tribune has a six-strong editoral team, including deputy editor Barckley Sumner.
Previous editors at the title, which launched in the 1930s to support anti-fascists during the Spanish Civil War, have included NHS founder Aneurin Bevan and former Labour leader Michael Foot. Author George Orwell also worked at the paper as literary editor during the 1940s.
Last month, a new left-of-centre magazine called Underground Focus was launched to compete with Tribune and other left-wing magazines in the market including New Statesman and Red Pepper.
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