Suroush Saraie has been held in Adel Abad Prison in Shiraz since 5th July 2014, following a raid by the security services on the home of a Church of Iran pastor.
He has now been released 14 months early.
Mr Saraie was first arrested in October 2012 along with six other Christians during a raid on a prayer meeting and found guilty of "action against the national security" and "propaganda against the order of the system".
He was given a two-and-a-half year sentence in July 2013, which was upheld on a later appeal.
Christian Solidarity Worldwide (CSW) is welcoming his release and claims that human rights have gotten worse since President Hassan Rouhani took to power in 2013.
CSW claims that political opponents, journalists, activists and members of religious minorities continue to be imprisoned, with converts to Christianity particularly targeted.
President Rouhani is due to visit Italy in November where he will be meeting with Pope Francis.
CSW's Chief Executive Mervyn Thomas said: "While we welcome Suroush Saraie's early release, we note that he should never have been detained and charged with security crimes for simply exercising his right to freedom of religion or belief by gathering peacefully with fellow Christians.
"Contrary to the views expressed at the meeting at St George's in Tehran, the Special Rapporteur's findings, which confirm CSW's research, indicate a worrying deterioration in the human rights situation in Iran.
"We therefore urge the Iranian authorities to release the many other prisoners of conscience who are unjustly detained and to ensure that constitutional guarantees for religious freedom become a reality for all of Iran's religious communities."
