The exact details relating to his release are not known.
Makan was arrested on June 26. Amnesty said he had been ill treated while in custody.
Amnesty says Makan told his family that if he signs a "confession" saying that the banned People's Mujahedin Organization of Iran (PMOI) killed Neda, then he could be released. Amnesty had said it feared he may be forced to sign such a "confession" under torture or other ill treatment.
It was reported in the Iranian press that Makan had denied that he had been required to "confess” prior to being released.
Despite being released, Amnesty says it is concerned that Makan remains under threat and will continue to monitor his case.
Neda, 27, was killed by a gunshot to the heart on June 20. Video footage of Neda's death on a Tehran street was widely circulated on the Internet and became a symbol of the opposition protests.