Independent Election Commission chairman Azizullah Lodin announced Karzai as the victor during a news conference in Kabul on Monday.
President Hamid Karzai had already effectively secured a second term when his only challenger, former Foreign Minister Abdullah Abdullah, dropped out of the race Sunday because he said the vote would not be free or fair. The first round was marred by massive fraud.
Meanwhile, the U.N. chief made a surprise visit to Afghanistan on Monday as international pressure mounted for a quick resolution to the country's electoral turmoil following the withdrawal of President Hamid Karzai's only challenger.
U.N. Secretary-General Ban Ki-moon was to meet with the two men "to assure them and the Afghan people of the continuing support of the United Nations toward the development of the country," the statement said.