Police in Tak have arrested 15 Rohingya Muslims on charges of illegal entry and are investigating the possibility that they could be victims of human trafficking.
Twelve of the detainees were arrested late on Tuesday and three more on Wednesday morning in Mae Sot, close to a bridge between Thailand and Myanmar, police said.
All were from Buthidaung, home to one of the largest concentrations of Rohingya Muslims in Myanmar's northwestern Rakhine State.
"They have been preliminarily charged with illegal entry, pending further interrogation to determine whether they are victims of human trafficking," Phubed Sang-aram, superintendent of the Mae Sot district police, told Reuters by phone.
Mr Phubed said three Thais had also been arrested for providing accommodation to illegal migrants.
Local television showed video of the group of mostly Rohingya women sitting on the floor being questioned by police.
The group had planned to go on to Malaysia, said Chaiyapheuk Chiantranluk, the Mae Sot district chief.
Vast numbers of Rohingya fled a 2017 military crackdown in the largely Buddhist Rakhine state that the UN said was executed with genocidal intent.
Many Rohingya risked death and starvation on perilous boat journeys in the hope of reaching predominantly Muslim Malaysia.
Myanmar denies allegations that its security forces have committed atrocities against them and says it was conducting a legitimate security operation against militants who attacked police posts.
It says Rohingya are illegal immigrants from South Asia. More than a million of them now live in camps in neighbouring Bangladesh, too frightened to return.