Soumaila Cisse, the rival candidate, conceded his loss and announced on Twitter late Monday he had visited Keita to offer his congratulations, the Los Angeles Times reported.
Marie Harf, a spokeswoman for the U.S. State Department, said the United States plans to resume aid to Mali. At a news conference Monday in Washington, she congratulated the West African country on holding a democratic election.
"So despite heavy rains, Malians took to the polls in large numbers yesterday to peacefully cast their vote for the next president of Mali," she said. "We commend the Malian people for their enthusiasm and engagement in this election."
In the first round of the election, Keita got 40 percent of the vote to 19 percent for Cisse.
Mali is a landlocked country with much of the north in the Sahara Desert. In 2012, Tuareg rebels in the north launched a military operation that was followed by an al-Qaida takeover of parts of the north. In January, France launched a military intervention to support the government against both groups.