Marziyeh Afkham described the peaceful elections and the presidential candidates' respect for the results as signs of good and admirable understanding among the Nigerian political parties and politicians of the “importance of the situation and the priorities in this great country”.
For the first time in Nigeria's history, the opposition defeated the ruling party in democratic elections.
Buhari defeated incumbent Goodluck Jonathan by about 2 million votes, according to Nigeria's Independent National Electoral Commission.
Earlier, Jonathan phoned Buhari to concede defeat. The outgoing president also offered a written statement to his nation.
"I thank all Nigerians once again for the great opportunity I was given to lead this country, and assure you that I will continue to do my best at the helm of national affairs until the end of my tenure," Jonathan said.
Buhari, 72, will be sworn in on May 29. He will take the helm at a critical time, as Nigeria grapples with Boko Haram terrorists, serious economic woes and corruption.