Hu, 34, a high-profile rights campaigner well known for criticising the government in internet articles and in interviews with the foreign media,
was jailed for "incitement to subvert state power".
RSF also urged the European Union to freeze its rights dialogue with China to protest against the prison sentence.
Condoleezza Rice, the US secretary of state, said on the sidelines of the Nato summit in Romania on Thursday that the decision "is deeply disturbing to the United States".
"It is exactly the kind of decision that we have tried to convince the Chinese is not only not in the interest of human rights, and in the interest of the rule of law, but actually not in China's interest," she said.
Calling Hu "a figurehead of the peaceful struggle to improve respect for human rights in China", the RSF said "the list of Olympic Games prisoners is getting longer while the International Olympic Committee remains desperately silent".
"In a sign of protest, we urge the European governments to immediately freeze the constructive dialogue on human rights that has been conducted with China for the past few years," the group said.
Rule of law
A Chinese foreign ministry spokeswoman said the case "has been dealt with in accordance with law".
"We will not stop the implementation of law because of the Olympic Games," Jiang Yu, the spokeswoman, said.
Hu, who has been involved in civil liberties issues ranging from Aids awareness to environmental rights and Tibet, was found guilty less than two weeks ago and has 10 days to appeal Thursday's sentence.
Last year Hu and activist lawyer Teng Biao authored an article accusing Beijing of failing to fulfil a pledge made when bidding for the Olympics that it would improve human rights.
Another government critic, Yang Chunlin, was jailed five years on similar charges last month.