May had ruled out any head-to-head debate with other party leaders ahead of the June 8 vote.
May had ruled out any head-to-head debate with other party leaders ahead of the June 8 vote, and aides confirmed her interior minister would attend for the ruling Conservatives as planned.
Labour leader Corbyn had previously said that he would not attend the seven-way clash on Wednesday if May was absent -- only to announce hours before the event that he would take part after all.
He challenged May to join him in Cambridge, eastern England, accusing her of running scared.
"I will be taking part in tonight's debate because I believe we must give people the chance to hear and engage with the leaders of the main parties before they vote," he announced.
"The Tories have been conducting a stage-managed arms-length campaign and have treated the public with contempt.
"Refusing to join me in Cambridge tonight would be another sign of Theresa May's weakness, not strength."
Questioned at a rally in Bath, western England, May said that "debates where the politicians are squabbling among themselves doesn't do anything for the process of electioneering".
She said her campaign was "about getting out and about, meeting voters and hearing directly from voters".
Labour is gaining ground on the Conservatives in opinion polls, and Corbyn's team has been buoyed by a better-than-expected performance in a TV grilling on Monday night.
The leaders of the Liberal Democrats, the UK Independence Party, the Greens and Welsh nationalist Plaid Cymru, and the deputy leader of the Scottish National Party, are also taking part in the debate.
Greens co-leader Caroline Lucas said the prime minister's failure to attend showed "extreme cowardice".