P. Devakumar, 36, a correspondent for the privately-run Sirasa television, was murdered on Wednesday, the influential Free Media Movement (FMM) said.
Devakumar is the ninth media worker to have been killed in Jaffna since 2006, the FMM said.
Three more journalists have also been abducted in the town over the last two years.
"It is with sickeningly increasing frequency that we are compelled to ask the government to take concrete measures to halt the killing, assault and intimidation of journalists in Sri Lanka," FMM said in a statement.
Jaffna, the main town on the embattled northern peninsula of the same name, has been under military control for over a decade.
Media rights watchdogs describe Sri Lanka as one of the most dangerous places in the world for journalists after Iraq.
At least 10 Sri Lankan media workers have been killed over the past two years, while others have been abducted, tortured or illegally detained, according to Amnesty International.
Most are Tamil journalist