It ends months of speculation about whether the 74-year-old leader plans to run for re-election.
His first term has been beset by poor health, which saw him spend months in the UK last year receiving treatment.
Mr Buhari defeated former President Goodluck Jonathan in the 2015 election.
He was the first opposition leader to defeat an incumbent in Nigeria.
The announcement comes as he is due to travel to the UK on an official visit.
He is scheduled to meet UK Prime Minister Theresa May, and attend the Commonwealth Heads of Government Meeting which starts on 16 April.
Mr Buhari was on "medical leave" in the UK for three months early last year.
He revealed after his return to Nigeria that "I have never been so sick", but did not disclose what he was suffering from.
Mr Buhari will run under the banner of the ruling All Progressives Congress. The main opposition People's Democratic Party is yet to announce its candidate.
He has been under fire from former President Olusegun Obasanjo, who in an open letter called on him not to seek re-election because of his age and alleged poor health.
Mr Obasanjo added that he was disappointed with Mr Buhari, particularly because of what he called his poor handling of Nigeria's economy, the largest in Africa.
Mr Buhari's spokesman said the president accepted the criticism in good faith, but it should be noted that significant progress had been made under his rule in tackling Nigeria's problems.
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