According to the Paris-based Tunisia Solidarity Movement, Abd Allah Zuari, 48, has lost 9kg in the last month and is dangerously weak.
Zuari, who wrote for a newspaper considered sympathetic to an Islamist opposition group, was sentenced to 11 years in prison in 1990 and since 2001 has been under virtual house arrest.
He is currently being held in a remote village on the edge of the Sahara desert.
In a statement on Wednesday, the Tunisia Solidarity Movement quoted a doctor who examined Zuari as saying his "medical situation is high risk and without an urgent end to his hunger strike his condition will deteriorate unpredictably, even dramatically".
The organisation said the Tunisian government is responsible for Zuari's plight and its treatment of him violates accepted human rights standards.
But it called on Zuari to end his hunger strike because "a living journalist is much more useful to the cause of liberty than a dead one".
Muzzling the press
Zuari's hospitalisation comes two days after six members of his family went on hunger strike to protest against government restrictions they say are preventing him working and living a normal life.
Fathi Enneas, the secretary-general of the Tunisia Solidarity Movement, said: "It is sad to note that this family has to go on hunger strike just to claim their natural right to live the normal life which they have been deprived of for 13 years."
"[Zuari's] medical situation is high risk and without an urgent end to his hunger strike his condition will deteriorate unpredictably, even dramatically"
Doctor quoted by the Tunisia Solidarity Movement
He added: "We renew our appeal for more solidarity with this journalist and his martyr family in a country which has nevertheless received the blessings of the world's democracies."
is North Africa's most stable and prosperous country, but human rights groups have repeatedly accused the government of muzzling the press, beating dissidents and restricting political life.
A report issued by the International Federation of Journalists on Tuesday warned that freedom of expression is under siege in Tunisia.
The world's largest journalists' group said human rights activists are being intimidated, dissident internet sites closed down, and books and newspapers censored.
The report said there is extensive police surveillance of email messages and internet cafes and that individuals are detained for expressing their opinions.
Tunisia angrily rejected the accusations on Wednesday, saying the report was seriously flawed.
"Opposition newspapers publish regularly and express themselves on all issues of the day. The media are free in Tunisia and there is absolutely no press censorship. Media freedoms are protected by law and in practice"
Tunisian government statement
"[The report's] authors have not properly checked their information and their conclusions therefore lack credibility," a government statement said.
"Freedom of expression is a reality in Tunisia. Members of the opposition express their views freely and without any restrictions, whether in parliament where five opposition parties are represented or in public forums.
"Opposition newspapers publish regularly and express themselves on all issues of the day. The media are free in Tunisia and there is absolutely no press censorship. Media freedoms are protected by law and in practice," it said.
The statement denied "the beating of dissidents", saying that "the rule of the law is constantly upheld in Tunisia and the perpetrator of any proven form of mistreatment is held accountable".
It did not comment on Zuari's case.