According to Christian Solidarity Worldwide, Mohammad Ali Dadkhah was imprisoned last Saturday.
He was the lead advocate in the case against Pastor Nadarkhani, who was sentenced to death for apostasy and eventually freed last month after he was acquitted by a judge.
Mr Dadkhah was first arrested in July 2009, along with his daughter and three of his associates, and was accused of “keeping weapons, opium as well as documents” that illustrated links with foreign “enemies”. He was released in September that year after posting bail of around $500,000.
In July last year, the 15th Branch of the Revolutionary Tribunal disbarred Mr Dadkhah for “actions and propaganda against the Islamic regime”, and for keeping banned books in his home.
In addition to a 10-year ban on practising law or teaching at the university, Mr Dadkhah was sentenced to nine years in prison.
Mr Dadkhah is co-founder of the Defenders of Human Rights Centre (DHRC) and has represented religious minorities, as well as political and human rights activists.
The DHRC was shut down by the authorities in 2008 and former members have reported ongoing harassment.
When his prison sentence was handed down last year, Mr Dadkhah said at the time: “I thank God that I did not get a worse sentence than this...I am not at all worried about these sentences, because I have acted lawfully and in defence of my homeland, and it is natural for someone who takes a step in the direction of law and order to be treated badly by those who are against law and order.”
CSW criticised the "assault" on civil society and the detentions of dozens of journalists, activists and human rights lawyers.
It warned that the crackdown on human rights lawyers in particular was reducing the availability of legal representation for those facing charges related to freedom of thought, conscience or belief.
CSW’s Advocacy Director Andrew Johnston called for Mr Dadkhah's release.
“The imprisonment of Mr Dadkhah is a clear miscarriage of justice and further proof that Iran’s campaign against civil society continues to gather pace," he said.
"Mr Dadkhah has clearly been victimised because of his diligence in ensuring due process for political and human rights activists facing trumped up charges, and this renders his conviction and imprisonment an even greater travesty.
"The persecution of human rights defenders is a breach of Iran’s international obligations, and pressure must be maintained on the regime to respect international law and the rights of all of its citizens."