Gnanasundaram Kuhanathan, news editor of the Tamil-language daily Uthayan, was attacked by a group of men wielding iron bars on Friday near his home in the area formerly held by separatist Tamil rebels, police said.
Kuhanathan was being treated in hospital for serious head injuries, police spokesperson Prishantha Jayakody told AFP, adding police were probing the assault.
"The man is in intensive care in hospital," Jayakody said. Police said no arrests had so far been made in connection with the attack.
At least four journalists working for the Uthayan newspaper have been killed since May 2006 because of its alleged pro-nationalist Tamil stance.
A total of 17 journalists and media employees have been killed in Sri Lanka in the past decade, with none of the murders being solved.
"For too long Sri Lankan authorities have been indifferent about the targeted attacks, killings and disappearances endured by journalists," said the New York-based Committee to Protect Journalists (CPJ) in a statement.
"That must end," said CPJ deputy director Robert Mahoney.
Attacks against journalists and news outlets have continued despite the end of a decades-long war between the Sri Lankan military and separatist Tamil Tiger rebels in May 2009, said Mahoney.
"We are extremely concerned for Gnanasundaram Kuhanathan and hope for his speedy recovery," he added.
The assault came a day after police found the body of Pattani Razeek, a human rights activist who disappeared in eastern Sri Lanka in February 2010.
The UN High Commissioner for Human Rights on Friday urged Sri Lanka to "expedite" investigations and prosecute those involved in the crime.
UN records show there are 5 653 outstanding "enforced and involuntary disappearances" in Sri Lanka.
Among those missing is Sri Lankan freelance journalist and cartoonist Prageeth Ekneligoda, who vanished on the eve of the January 2010 presidential election.