Sri Lanka's crackdown on the Tamil Tigers, which ended a lengthy civil war last year, has resulted in a flood of people seeking asylum in other countries.
The Sri Lankans, who were rounded up at a crowded apartment in a Bangkok suburb, appear to be economic migrants hoping to travel to a third country, said Police Major General Manu Mekmok, of Thailand's Immigration Bureau.
"They either overstayed their visas or entered illegally," he said.
In October Thai authorities detained more than 200 Tamil migrants from Sri Lanka in two separate operations, but they later released some who were able to present valid travel documents.
Some reports at the time suggested those Sri Lankans hoped to travel to Canada, whose government welcomed news of their arrest.
Almost 500 Tamil refugees arrived in Canada in August aboard a cargo ship which reportedly spent 90 days travelling from Thailand before police boarded it in Canadian waters and piloted it ashore in western Canada.