"Ernest Bai Koroma has been duly elected president of the Republic of Sierra Leone," NEC chairperson Christiana Thorpe told a news conference in Freetown.
The NEC declared Koroma the winner despite an announcement by the SLPP at the weekend that it would seek a court injunction to prevent any more results from the tense presidential run-off being released because it said those declared so far were not credible.
The election was seen as a test of the former British colony's recovery from a 1991-2002 civil war, one of modern Africa's most brutal in which 50,000 people were killed and children were kidnapped, drugged and forced to fight.
The polls were the first since United Nations peacekeepers left two years ago.
Though rich in diamonds, Sierra Leone has a citizenry that is poor and unemployed, and corruption is rampant.
President Ahmed Tejan Kabbah was barred by law from running for a third five-year term.
Koroma, 54, led the first round of voting on August 11, winning 44 per cent of the vote, compared with 38 per cent for Berewa, 69.
However, that margin was not large enough to win outright.