Guterres “applauds all Liberians for the successful completion of the elections process, which was conducted in a peaceful environment,” a statement from the UN chief’s spokesman said.
Liberia’s election board certified Weah as the winner of the election on Friday, marking the country’s first peaceful transfer of power after two civil wars.
Weah, a former international football star, defeated Vice President Joseph Boakai in the run-off vote.
Weah is due to be sworn in on January 22, following Ellen Johnson Sirleaf, who in 2006 took over the country founded by freed US slaves.
Boakai conceded defeat on Friday and said he had called Weah to congratulate him.