Okah was jailed in 2013 for masterminding a number of attacks, including twin bombings which killed 12 people in the Nigerian capital Abuja during Independence Day celebrations in 2010.
Okah’s appeal was first heard by the Supreme Court of Appeal in 2014, which overturned four of his 13 terror convictions and reduced his sentence to 20 years, but the Constitutional Court in Johannesburg overruled the decision.
“The appeal against his entire conviction… is dismissed,” the Constitutional Court said in its ruling.
“The convictions overturned in the Supreme Court of Appeal must be reinstated together with the sentences the High Court imposed.”
Okah had repeatedly denied any involvement in the blasts.
Lawyers for Okah, who has been a permanent resident of South Africa since 2007, had argued that it was outside the country’s legal jurisdiction to put him on trial.
“He was not extradited to Nigeria because, had he stood trial there, he would have faced the death penalty,” said the Constitutional Court in its judgement.
Okah, who was also convicted over two other explosions in 2010 in the southern Nigerian city of Warri, a hub of the oil-rich delta region, was the presumed leader of the Nigerian militant group Movement for the Emancipation of the Niger Delta, MEND.
MEND said it was fighting for a fair share of oil revenue for residents of the Niger Delta.
The group has seen spin-offs by radicals who want independence for the region.
A high court in Abuja is expected to hand down a ruling on March 7 in the trial of Okah’s brother Charles for his alleged involvement in the 2010 Independence Day bombings.