The survey commissioned by the BBC found that 52% disagreed with the statement that "the internet is a safe place to express my opinions".
Researchers also found that one in three people do not feel free from government surveillance, with US citizens the most concerned about snooping in the wake of the National Security Agency scandal.
Belief in media freedom has also fallen in recent years, according to the study. Since 2007, internet user confidence that the media in their country has the freedom to report accurately has fallen by 19%.
The findings come after two people were jailed earlier this year for sending death threats to a journalist campaigning to have Jane Austen placed on the new £5 note.
Tony Neate, chief executive of getsafeonline.org - a government support group for using the internet safely - said: "The internet is a fantastic forum for discussion and debate, enabling us to connect with people from nations all over the world and hear differing opinions.
"In the UK we are fortunate enough to enjoy freedom of expression but this isn't the case in all countries."
Last week, the Turkish government banned YouTube from the country, following a similar move with social network Twitter, after it claimed the sites were unfairly spreading anti-government propaganda.
The country's journalist union and bar association responded with court action citing a right to freedom of expression after audio files were leaked online allegedly linking government officials to corruption.
While Turks have found ways around the ban, neither is yet to be lifted, despite a court ruling against the government on the Twitter ban.