He joined the race as thousands of opposition party supporters marched to UN peacekeeping headquarters to demand renewed registration so they can vote.
"I'm joining the race as the candidate of the people," said Kabila, the 34-year-old leader of Congo's postwar national-unity government, which is arranging the first presidential elections in four decades.
Supporters of opposition leader Etienne Tshisekedi have repeatedly raised the registration issue in recent weeks as Kofi Annan, the UN secretry-general, visits their country.
Annan, who was not present at UN headquarters during the march, said later that the vote shouldn't be delayed. "The people of Congo have had enough. They're tired of waiting."
Tshisekedi, a veteran Congo opposition leader, called on his supporters last year to boycott registration for 18 June elections arranged by Kabila's transitional administration.
Tshisekedi later reversed himself, saying he would run. Now he is demanding election centres across the vast, war-ruined nations re-open to register his supporters.
Later on Wednesday, The European Union agreed in principle to a German-led military mission of up to 1450 troops that will help protect the Congolese elections, EU diplomats said.
The deployment would take place two to three weeks before the elections and troops would stay for about four months, the official added.