New measures introduced in December were being deployed against those who question the government's handling of the separatist war with the rebel Liberation Tigers of Tamil Eelam (LTTE), the group said in a statement.
It said the Sri Lankan Standard Newspapers, which published a leading Sinhalese-language weekly and an English-language weekly, stopped operations after the government froze their assets, citing suspected links to the LTTE.
An accountant of the company was arrested under the tough Prevention of Terrorism Act which allows detention up to a year without formal charges being brought.
"The government is using anti-terrorism legislation to silence the press," said HRW Asia research director Sam Zarifi said.
"As the war heats up, the government is clamping down on criticism and dissent."
Tamil-language media have often come under pressure from the Sinhalese-dominated government, but it is the first time in three decades that Sri Lankan authorities shut down a Sinhalese lan