The official swearing-in ceremony was held during a special session of Cameroon's National Assembly in the capital, Yaounde.
Biya was administered the oath of office by Cameroon's speaker of the National Assembly, Cavaye Yeguie Djibril.
"Thank you for accepting the verdict of the poll. That is democracy." Djibril told members of parliament and guests present at the ceremony.
"All Cameroonians recognize you as the president of the republic. With you Cameroonians live in peace and freedom. You are the right man at the right place," Djibril told Biya before administering the oath of office.
Cameroon's Constitutional Council declared Paul Biya winner of the Oct. 7 presidential election with 71.28 percent of the votes. Biya has been in office since 1982. He will serve the country for another seven years.