China was next after Iran with 24 jailed journalists and then Cuba with 22. The number of journalists held in Iran was the highest recorded by the New York-based CPJ in a single country since 78 cases were documented in Turkey in 1996.
Several publications in Iran have been banned and many journalists detained since street protests broke out in the aftermath of presidential elections last year.
The CPJ said the number of journalists jailed in Iran rose by five in February from January after 12 members of the media were imprisoned and then seven of them were released.
Of the 52 journalists in jail, five had been held since before the crackdown began last year, the CPJ said. Another 50 journalists have been imprisoned and released on bail during the past several months.
"Iran is entering a state of permanent media repression, a situation that is not only appalling but also untenable," CPJ Executive Director Joel Simon said.
"The Iranian government will eventually lose the war against information, but we are saddened every day that our colleagues are paying such a terrible price."
The disputed re-election of President Mahmoud Ahmadinejad in June 2009 plunged the Islamic Republic into its deepest internal crisis in its three-decade history and created a rift within the ruling establishment.
Reformist opposition leaders and their supporters say the poll was rigged to secure Ahmadinejad's re-election, an allegation the authorities deny.
Hardliners accused opposition leaders Mirhossein Mousavi and Mehdi Karoubi of inciting unrest and called them "enemies of God" — a crime punishable by death under Iran's Islamic law.
(Reporting by Michelle Nichols; Editing by John O'Callaghan)
