Held since June 30 without formal charges, Ibrahim expected a 15-day detention period to be extended a third time.
Egyptian law allows a person to be imprisoned six months without charges.
Nadia Abdel-Nour, an assistant at the Khaldoun center, also was released, Barbara Ibrahim said. Saad Eddin Ibrahim is known internationally for his work on democracy, Islamic fundamentalism and elections monitoring in the Arab world. Human-rights groups such as Amnesty International and Human Rights Watch condemned his detention. On Monday, his lawyers said Ibrahim was being accused of spying for the United States. Human-rights activists applauded Ibrahim's release, but said his arrest damaged Egypt's image abroad.