Bemba arrived in a private jet in the capital, Kinshasa, to throngs of supporters that included members of his opposition Movement for the Liberation of Congo party. The police presence was high.
His arrival shakes up the political landscape in one of Africa's most turbulent nations as the opposition worries that President Joseph Kabila, whose mandate ended in late 2016, will find a way to hold onto power.
Kabila has been in power since 2001, and his government has blamed delays on the difficulties of organising a vote in the vast country.
While the international community’s patience frays - the US last week warned Kabila that “the time for posturing is over” - Congo’s president has remained quiet on his role in the upcoming election and declared that his country is rejecting foreign meddling and funding the vote itself.
Bemba, like other candidates, must submit his candidacy by August 8 to run in the elections now set for December 23, more than two years late. Bemba was granted a diplomatic passport to return to Congo because he is still a senator.
In an interview over the weekend, Bemba said he wanted Congo’s opposition to rally around a single challenger to Kabila and would “give all my support” if that challenger isn't him.
Just weeks ago, few had suspected Bemba would be in a position to compete.
In a surprising ruling in June, appeals judges with the ICC overturned Bemba’s convictions for murder, rape and pillaging committed by his Movement for the Liberation of Congo forces in neighbouring Central African Republic in 2002 and 2003.
He had been sentenced in 2016 to 18 years in prison but maintained his innocence. He is awaiting a final sentencing at the ICC in another case in which he was convicted of interfering with witnesses.
Another popular opposition leader, Moise Katumbi, has promised to return to Congo this week to stand in elections. Demonstrations over Congo’s election delay have turned deadly in the past, and Pope Francis and others appealed for calm.