Independent MP Zarah Sultana said she and the former Labour leader will co-lead the new party, which she did not provide a name for.
She said other independent MPs, campaigners and activists from across the country will join them, but did not name anyone.
Ms Sultana also said she was “resigning” from the Labour Party after 14 years.
She was suspended as a Labour MP shortly after they came to power last summer for voting against the government maintaining the two-child benefit cap.
Several others from the left of the party, including Mr Corbyn, were also suspended for voting against the government, and also remained as independent MPs.
However, Ms Sultana was still a member of the Labour Party – until now.
She said she would vote to abolish the two-child benefit cap again and also voted against scrapping the winter fuel payment for most pensioners.
Ms Sultana also voted against the government’s welfare bill this week, which was heavily watered down as Sir Keir Starmer tried to prevent a major rebellion from his own MPs.
On Wednesday, Ms Sultana spoke passionately against Palestine Action being proscribed as a terror organisation – but MPs eventually voted for it to be.
She said to proscribe it is “a deliberate distortion of the law to chill dissent, criminalise solidarity and suppress the truth”.
Ms Sultana said they were founding the new party because “Westminster is broken but the real crisis is deeper – just 50 families now own more wealth than half the UK population”.
She called Reform leader Nigel Farage “a billionaire-backed grifter” leading the polls “because Labour has completely failed to improve people’s lives.
The MP, who has spoken passionately about Gaza, added: “Across the political establishment, from Farage to Starmer, they smear people of conscience trying to stop a genocide in Gaza as terrorists.
“But the truth is clear: this government is an active participant in genocide. And the British people oppose it.
“We are not going to take this anymore.”
A Labour Party spokesperson said: “In just 12 months, this Labour government has boosted wages, delivered an extra four million NHS appointments, opened 750 free breakfast clubs, secured three trade deals and four interest rate cuts lowering mortgage payments for millions.
“Only Labour can deliver the change needed to renew Britain.”
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