Deputy Prime Minister Uhuru Kenyatta and former minister William Ruto addressed thousands of supporters in western Kenya to seal their pact.
The ICC has indicted the two over the violence that hit Kenya after the disputed 2007 elections.
Their case is due to start after the March election.
At a joint rally in the Rift Valley town of Nakuru, the two leaders announced that Mr Kenyatta would be the presidential candidate and Mr Ruto his running mate.
"We have agreed we are uniting on behalf of the people of Kenya. Our alliance is not for fighting anyone. We are uniting for the sake of the people of Kenya," Mr Kenyatta said.
Mr Ruto said they were not concerned about the ICC case.
"Those talking ill of Ruto and Uhuru are wasting their time. The ICC cases have formed the basis of debates but we are going to prove them wrong [and show] that Kenyans are united and want peace," he said.
Both men deny playing key roles in the violence which followed the 2007 poll.
Some 1,200 people were killed and 300,000 forced from their homes.
Mr Kenyatta and Mr Ruto were on opposite sides in the 2007 election.
Mr Kenyatta backed President Mwai Kibaki, while Mr Ruto was part of Raila Odinga's alliance seeking to oust the president.
Both men have previously said they would contest the presidential election despite the ICC charges.
Mr Odinga, who became prime minister under the deal brokered to end the bloodshed, is also contesting the presidential election.
Mr Kibaki is stepping down next year at the end of his second and final term.
Two bitter political rivals in Kenya facing trial at the International Criminal Court have confirmed their alliance for next year's election.