"Control of news and information continued to tempt governments and to be a question of survival for totalitarian and repressive regimes," said the Paris-based group.
RSF said it was no surprise that the same trio of countries -- Eritrea, North Korea and Turkmenistan -- were bottom of the list because they were "absolute dictatorships that permit no civil liberties".
"They are immediately preceded at the bottom by Syria, Iran and China, three countries that seem to have lost contact with reality as they have been sucked into an insane spiral of terror," it said.
Bahrain and Vietnam -- both described as "quintessential oppressive regimes" -- were also down at the bottom, while RSF said "other countries such as Uganda and Belarus have also become much more repressive".
Tunisia rose 30 places from last year's index to 134th but "has not yet fully accepted a free and independent press", according to RSF.
Bahrain, now ranked 173rd, fell 29 places because of its "relentless crackdown on pro-democracy movements, its trials of human rights defenders and its suppression of all space for freedom," the group said.
Egypt fell 39 places to 166th "because the Supreme Council of the Armed Forces, in power since February, dashed the hopes of democrats by continuing the (ousted president Hosni) Mubarak dictatorship's practices."
"Total censorship, widespread surveillance, indiscriminate violence and government manipulation made it impossible for journalists to work" in Syria last year, which fell to 176th position in the index.
Elsewhere, pro-democracy movements that tried to follow the Arab example were ruthlessly suppressed, with, for example, many arrests made in Vietnam (172nd), said RSF.
In China, which ranks 174th, the government responded to regional and local protests and to public impatience with scandals by feverishly reinforcing its system of controlling news and information, it said.
China carried out extrajudicial arrests and stepped up internet censorship, it added.
In Azerbaijan (162nd), there was a dramatic rise in the number of arrests, as the government jailed netizens, abducted opposition journalists and barred foreign reporters in order to impose a news blackout on unrest, it said.
Led by President Yoweri Museveni, Uganda (139th) launched "an unprecedented crackdown on opposition movements and independent media after the elections in February".
Similarly, Chile (80th) fell 47 places because of its many freedom of information violations, committed very often by security forces during student protests.
The United States (47th) also owed its fall of 27 places to the many arrests of journalists covering Occupy Wall Street protests.
The index highlighted the divergence of some European countries from the rest of the continent.
The crackdown on protests after President Alexander Lukashenko's re-election caused Belarus to fall 14 places to 168th.
Turkey (148th) lost 10 places because it failed to carry out promised reforms and launched a wave of arrests of journalists that was without precedent since the military dictatorship, RSF said.
Within the European Union, the index reflected a continuation of the distinction between states like Finland and Netherlands that have always had a high ranking and states like Bulgaria (80th), Greece (70th) and Italy (61st).
RSF noted South Sudan among its "noteworthy changes" of 2011, pointing out that the new nation had entered the index in a respectable position (111th) for what is a breakaway from one of the worst ranked countries, Sudan (170th).
But it said that Africa also saw the biggest falls in the index.