Dokubo-Asari was arrested in the neighbouring country on Tuesday, his lawyer Festus Keyamo, said.
Keyamo confirmed the release on Thursday morning.
The ex-militant was said to have been released at about 1am Nigerian time on Thursday. It could not be confirmed, however, if the Niger Delta warlord was flown to Abuja in a presidential jet as sources at the airport said.
Keyamo had on Wednesday evening asked President Goodluck Jonathan to intervene and ensure the armed activist was released from Benin.
