"Mr Ouattara is the winner of the election," said Mutharika in a statement. "President Gbagbo should respect the will of the people through the ballot and therefore needs to stand down to avoid another bloodshed in Africa over power," he added.
Mutharika said he agreed with the 15-member regional bloc Economic Community of West African States (Ecowas) who suspended Côte d'Ivoire on Tuesday and called for Gbagbo to yield power.
French President Nicolas Sarkozy applauded the group's decision as "courageous" and "historic".
Despite calls and pressure from international observers for Gbagbo to step down, he remains installed in the presidential mansion.
Meanwhile, Guillaume Soro, the Prime Minister of Alassane Ouattara, said that Gbagbo "is not only a cheat but a dictator."