A South African court on Monday found Nigerian rebel leader Henry Okah guilty of masterminding two car bombings in Abuja in 2010 that killed 12 people.
Judge Neels Claassen in Johannesburg said Okah's decision not to testify during the trial left evidence against him uncontested and the prosecution had successfully proven he was involved in terrorist activities.
Okah was arrested in South Africa after the attacks which targeted crowds celebrating Nigeria's 50th year of independence from Britain.
The Nigerian Movement for the Emancipation of the Niger Delta (MEND) has distanced itself from Okah since the bombings, though his lawyers have denied he was involved.
Okah ended up in South Africa after accepting an amnesty deal offered by the Nigerian government to MEND.
The rebel group said it was defending the impoverished people of the Niger Delta who see little profit from the vast oil wealth.
It is alleged Okah continued to be involved in terrorist activity in Nigeria from his new base.
South Africa said it did not receive a formal extradition request from Nigeria and put Okah on trial domestically in accordance with local laws.
Sentencing is expected at the end of the month, according to the SAPA news agency.