The Foreign Affairs Ministry is warning Thais visiting South Korea against overstaying in the country to become illegal job seekers known as "little ghosts".
Natapanu Nopakun, deputy spokesman for the ministry, said in a statement on Friday that the number of Thai illegal workers in South Korea has reached nearly 140,000.
The statement was issued following incidents of South Korean immigration officers denying many Thais entry to the country after suspecting they may be illegally seeking employment.
In one incident on Jeju Island, about 455 kilometres from the capital Seoul, 55 Thais disappeared after flying to South Korea with a tour group. They are suspected to be illegal job seekers.
According to South Korea's Yonhap news agency on Sunday, 55 out of 280 Thai tourists who arrived on the resort island on a three-day tour package did not return to Thailand on Aug 5 with the others.
Immigration officers on the island were attempting to locate them, Yonhap said.
South Korean officials denied entry to 417 out of 697 Thais visiting Jeju from Aug 2 to Aug 5, including 112 on Aug 2, after they were suspected of being job seekers, not tourists, it said.
Mr Natapanu said to prevent such incidents from occurring again, Thais should not overstay their 90-day tourist visas.
Anyone who violates the rules will either have to pay a fine or be permanently banned from entering South Korea, he said.
Thai nationals who work illegally in South Korea will be detained and deported back to Thailand within one week, he said.
According to the Foreign Affairs Ministry, as of June, there were 181,783 Thais living in South Korea, 139,245 of whom were illegal workers while 532 were prisoners who had violated the law.
The Royal Thai Embassy in Seoul said illegal workers fully vaccinated against Covid-19 who have overstayed their visas will not be blacklisted if they leave South Korea before Oct 31.