He received almost 59% of votes cast, meaning there is no need for a run-off as he won more than 55%.
His main challenger, ex-military ruler Julius Maada Bio, took 38% of the ballots in Saturday's peaceful poll.
It was the third election since the 1991-2002 civil war, which killed more than 50,000 people.
International observers have already declared the election to be peaceful and transparent.
The average national turnout was 87.3%, the election commission said.
Mr Koroma, who faced eight candidates, took 1,314,881 votes - 58.7% of the total.
It will be his second and final term in office.
His supporters flooded the streets of the capital as news of his comfortable win became known - they danced, sang and banged pots and pans, the BBC's Mark Doyle reports from Freetown.
Parliamentary and local council election results are yet to be declared.
Saturday's vote was the first post-war election Sierra Leone had organised itself - the other two held since the war ended in 2001 were run by the United Nations.
Although many people around the world might still associate Sierra Leone with the Hollywood movie Blood Diamond - a place of war and atrocities - that image today could not be further from the truth, our correspondent says.
Sierra Leone is now a peaceful, democratic nation and the economy is growing fast - even if it remains one of the poorest nations in Africa, with a large proportion of the population of about six million living on less than $1.25 (80p) a day.
The army - once an undisciplined force containing a large number of rebels - has been rebuilt with considerable military aid from the UK and now sends peacekeeping soldiers to serve in UN missions around the world.
Sierra Leone's incumbent President Ernest Bai Koroma has won the presidential election, the election commission has said.