The rights group called on Iranian authorities to release "at least" 10 journalists arrested since the beginning of the year. The organization said it feared Iranian authorities were cracking down on free speech ahead of March parliamentary elections.
All of the journalists arrested this year were associated with reformist newspapers or wrote for Web sites critical of the government.
Joe Stork, deputy Middle East director at Human Rights Watch, said from New York that none of the journalists held by government authorities was charged with a crime.
"This wave of arrests against journalists and bloggers is a brazen attempt by the authorities to exercise absolute control over information available to the citizens," he added.
Several of those arrested are allegedly held in Iran's infamous Evin Prison, controlled by the Iranian military.
Iran closed media access to the outside world following violent demonstrations that followed the contested 2009 re-election of President Mahmoud Ahmadinejad. Several of his opponents remain under house arrest.
Human Rights Watch notes Iranian Intelligence Minister Heidar Moslehi ordered sweeping arrests after uncovering what he was a plot launched through social media sites to "further America's objectives."