The Paris-based Reporters Without Borders (RSF) said J.S. Tissainayagam has been held under an anti-terrorism law for more than a year for writing about civilians caught in fighting between troops and Tiger rebels.
Nadesapillai Vidyatharan, who edits the Tamil daily Sudar Oli, has been detained since February 26 over suspected links with the rebel fighters, the RSF said.
Vidyatharan's newspaper has been staunchly critical of the military campaign against the Tigers, who are cornered in the northeast of the island.
According to official figures, nine journalists have been killed in Sri Lanka in the past three years, including Lasantha Wickrematunga, editor of the Sunday Leader, who was shot by unknown gunmen in January as he drove to work.
His newspaper was fiercely critical of government policy.