The World Transformed event is taking place alongside the Labour party conference in Liverpool next week.
But in a statement, Momentum said the paper had “smeared” victims of the 1989 Hillsborough stadium disaster – in which 96 Liverpool fans died – and it was backing a long-running boycott of the title in the city.
A Momentum spokesperson said: “This year’s festival is in Liverpool where there is a 23-year-long campaign to boycott the Sun after they smeared Liverpool fans in the wake of the Hillsborough disaster, falsely claiming that they ‘picked pockets of victims’, ‘urinated on brave cops’ and ‘beat up PCs giving the kiss of life’.
“We stand with the people of Liverpool and all of the families who lost loved ones in the disaster and are still fighting for justice. This is why the boycott of The Sun extends to The World Transformed.”
In 2012 The Sun issued an “unreserved” apology for its Hillsborough coverage, describing a front page reliant on briefings by South Yorkshire Police as “without doubt the blackest day in this newspaper’s history”.
The Sun’s press team declined to comment on the Momentum ban.
In 2016, Labour chiefs banned copies of the paper from its party conference following complaints from Liverpool mayor Joe Anderson.