The Committee to Protect Journalists (CPJ) says at least 35 journalists are in prison in Iran.
CPJ said many of the jailed journalists are under immense physical and psychological pressure to confess to crimes they have not committed, including crimes punishable by death.
CPJ said an additional 18 journalists were free on short-term furloughs granted for the Iranian New Year and were expected to report back to prison.
CPJ Executive Director Joel Simon says they will continue to monitor the situation closely.
The CPJ report comes after news another Iranian journalist and human rights activist has been jailed.
Abolfazl Abedini has been sentenced to 11 years in jail, RFE/RL's Radio Farda reports.
Abedini, a member of the nongovernmental organization Human Rights Activists in Iran, was arrested on March 3 in the southwestern city of Ahvaz after his home was raided.
His family said he was beaten by members of Iran's Revolutionary Guards as he was taken away.
with agency reports