The appointment of Festus Keyamo was conveyed via an April 16 letter signed by Rotimi Amaechi, the minister of transport and director-general of the Buhari Campaign Organisation.
Mr Buhari announced last week Monday that he would seek reelection, shortly before departing for London.
The news of the appointment comes a day after Mr Buhari told British Prime Minister Theresa May that he was more concerned about governance than the 2019 elections.
A Senior Advocate of Nigeria, prominent for his rights activism, Mr Keyamo remains one of the most vocal voices backing the Buhari administration since 2015.
He is expected to make a formal statement about his appointment soon.