Cellou Dalein Diallo, a former prime minister and Conde’s main rival, joined other opposition candidates in alleging irregularities in the official poll results that showed Conde with 59.5% of the vote.
But constitutional court president Mohamed Lamine Bangoura said they had produced no evidence and declared Conde the winner.
Conde reacted by appealing to national unity.
“I dedicate this victory to all Guineans, whether they voted for me or not. Beyond our differences, it’s the Republic of Guinea that we need to put above our personal ambitions,” Conde said on his Facebook page.
Diallo condemned the court ruling but stopped short of calling for protests, as he has in the past.
“The members of the high court of our country preferred to place themselves at the service of a man and his ambitions rather than respecting their oath and defending the law,” he said in a statement.
Conde’s supporters celebrated in the streets of the capital Conakry, while police officers were deployed near Diallo’s house to head off possible unrest.
Conde’s actions have raised fears about a democratic backslide in Guinea, Africa’s top bauxite producer, and West Africa more generally.
Ivory Coast President Alassane Ouattara was declared the winner of an election last month after running for a disputed third term.
Reporting by Saliou Samb; Writing by Aaron Ross; Editing by Jason Neely, Clelia Oziel and Mike Harrison