The Independent National Electoral Commission (INEC) declared the 76-year-old winner of the election in the early hours of Wednesday.
Buhari beat the People's Democratic Party’s (PDP) Atiku Abubakar with a margin of just under four million votes.
Nigeria held its elections last Saturday, but the results were delayed following violence in some parts of the country.
Buhari's All Progressives Congress (APC) won in 19 out of the 36 states, while the PDP also won in 17 states.
INEC Chairman, Mahmood Yakubu, said the APC secured 56 percent of the electorate, equivalent to 15.2m votes.
Atiku and the PDP, however, managed to get 41 percent or 11.3m of the votes.
"Muhammadu Buhari ... is hereby declared winner," the INEC Chairman said.
Meanwhile, the PDP has rejected the result, alleging electoral malpractices, including vote-rigging.