Four judges ruled in favour of NASA's petition challenging the results, while two - Njoki Ndung'u and Jackton Ojwang - ruled against.
"Presidential election was not conducted in accordance to the Constitution.The results are null and void," Maraga said
This means that fresh elections must be held within 60 days after invalidation of the presidential poll.
Making the ruling on Friday, Chief Justice David Maraga said the IBEC failed, neglected or refused to conduct election in keeping with laws.
"We were satisfied that the election was not conducted as the constitution dictates," he said.
But he noted that the Court found no evidence of misconduct on the part of the third respondent who is President Uhuru Kenyatta.
Justices Jackton Ojwang' and Lucy Ndung'u dissented against the majority decision.
Ndung'u said the election was conducted in accordance with constitution.
Ojwang' said the petitioner did not have enough evidence.
NASA went to the Supreme Court to challenge the results of a presidential election, which it says was rigged.
The opposition wanted Uhuru's victory nullified. According to the IEBC, Uhuru won the August 8 election by 1.4 million votes.
Last night the judges, aided by 40 experts, were burning the midnight oil at Nairobi’s Windsor Hotel, writing the judgment.
In anticipation of sporadic violence after the ruling, security has been heightened with major deployments and anti-riot and crowd control dr
During their submissions, NASA lawyers said the election results declared were not based on evidence as they cited mismatch of results in Forms 34B and the IEBC online portal.
James Orengo and Otiende Amollo outlined the "glaring errors" in IEBC forms that led to an allegedly rigged election. Orengo said the forms issued to them and the Supreme Court do not match with their respective constituencies.
But Uhuru's lawyers asked the Supreme Court to saying they were based on hearsay but no evidence.
"Hold some arguments with a belt and suspender ... it is about the vote. Courts cannot demand voter certainty. The essence of this election is whether the voters voted," Ahmednasir Abdullahi said.
Raila also ran for the presidency in 2007 and 2013, losing both times and blaming fraud.
He went to court to challenge the 2013 election result - a decision that helped quell a repeat of violence then.
The Star| Nancy Agutu