Henry Okah was accused of being the mastermind behind two car bombings in the Nigerian capital of Abuja on Oct. 1, 2010, in which 12 people were killed and 36 injured, Nigeria's Channel Television reported.
He was arrested the next day in South Africa.
Judge Neels Claasen found Okah guilty on 13 counts ranging from conspiracy to commit terrorism to detonating explosives.
Okah had denied charges by Nigerian officials that he was the leader of the Movement for the Emancipation of the Niger Delta, which had claimed responsibility for the blasts.
Claasen said testimony had shown Okah was the "leader, planner, funder [and] supplier" of the bombs.
Okah did not testify but had denied involvement in the bombings.
South African prosecutor Shaun Abrahams said the ruling showed "there is no safe haven in South Africa," and African and foreign law enforcement agencies could work together.
Okah faces a minimum sentence of life in prison.