The attack on the Uthayan newspaper office was the fourth attack on the media since January in Sri Lanka's former war zone in the north of the country.
"Six people with masks attacked the office, vehicles and computers," said the publisher of Uthayan, E. Saravanapavan, who is a legislator for the Tamil National Alliance party, which was linked to the former ethnic Tamil separatist rebels.
Five workers including a branch manager were hurt, he said. Police and government officials were not immediately available for comment.
Political violence has eased since Sri Lanka's army crushed the Tamil rebellion in 2009, but international human rights groups say rule of law problems persist, including abductions and attacks on the media and government critics.
No arrests have been made in connection with the three earlier attacks on the media this year in the north, where a civil administration has yet to be fully restored since the end of the war.
"This is not only a threat to media freedom, it's a threat to the whole country. The government should take responsibility for this," said Sunil Jayasekara, of the Media Movement in Sri Lanka, press freedom group.