Saudi Arabia, which is on the group's "Enemies of the Internet" list and ranks 164 out of 179 on its 2014 World Press Freedom Index, has been particularly aggressive in policing the Internet, including by arresting those who post critical articles or comments, it said.
"Fearing the spread of the Arab Spring, the countries of the Arabian Peninsula have reinforced surveillance and control of the media, starting with the Internet, which has come to be a place where people express themselves with a freedom not found in the traditional media," the group said in a statement.
"As a result, the cyber-police of the Gulf monarchies are on the lookout for any online article, post or tweet critical of government policy."
Kuwait topped the Gulf Cooperation Council (GCC) countries on the press freedom list but dropped 14 points to rank 91st worldwide.
The GCC also includes Saudi Arabia, the United Arab Emirates, Bahrain, Qatar and Oman. Bahrain and Oman improved slightly on the list but the other four dropped.
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Last year Saudi censors paid particular attention to calls for lifting the conservative kingdom's controversial ban on women driving, a popular topic online.
Asharq Al-Awsat columnist Tariq al-Mubarak was arrested in October on various charges, some related to a column criticising the ban.
In July, a court sentenced the founder of the now-censored Saudi Liberals website, Raef Badawi, to seven years in jail and 600 lashes, after he posted an article about St. Valentine that allegedly denigrated the religious police, RSF said.
In Kuwait, authorities have zeroed in on any perceived criticism of Islam, and a number of tweeters have been jailed for posting comments deemed offensive to the emir.
In Oman, discussion of the long-ruling sultan is considered taboo, and netizens have been given long jail terms, although some have subsequently been pardoned.
Bahrain's monarchy has cracked down on coverage of protests related to the Arab Spring uprising which was crushed almost exactly three years ago.
And the UAE has handed long jail sentences to two people who tweeted about the trial of scores of Emiratis accused of belonging to a local party linked to Egypt's Muslim Brotherhood, RSF said.