The group, which grew out of Jeremy Corbyn’s campaign to become Labour leader, said it had mobilised thousands of members and used social media to secure seats previously considered “unwinnable”.
New Sheffield Hallam MP Jared O’Mara said: “The contribution of Momentum members in South Yorkshire and beyond was exemplary.
“It was a blessing to have them on board campaigning to get me elected.”
Mr O’Mara’s victory brought an end to Mr Clegg’s 12 years as MP for the constituency.
A Lib Dem source familiar with the campaign ridiculed Momentum’s claim, pointing to the rise in the Conservative vote at the expense of the Lib Dems as a major factor in Mr Clegg losing his Commons seat.