Thai Airways International now requires passengers departing from China, Italy and South Korea to present health certificates before they are issued boarding passes.
THAI president Sumeth Damrongchaitham announced the restriction on the airline's Facebook page late on Monday night.
The statement said passengers joining flights in areas seriously hit by coronavirus disease 2019 (Covid-19) must present the health certificates to prove they are free of the disease.
THAI staff would issue boarding passes only to passengers with certificates, he said. This was in compliance with an instruction from the Civil Aviation Authority of Thailand.
The measure applies to passengers departing from all airports in virus-affected countries that THAI planes visit. It currently applies to Seoul and Busan in South Korea; Beijing, Chengdu, Guangzhou, Hong Kong and Shanghai in China; and Milan and Rome in Italy, Mr Sumeth said.