The Malaysian government is yet to confirm where or when he contracted the virus.
The 46-year-old Malaysian man was symptomless when he returned to Malaysia on August 5 but the result of a second test on August 15 revealed he was infected with Covid-19.
Health Ministry officials are investigating an unnamed condominium in Bangkok where the man is thought to have stayed and are preparing to screen residents there for Covid-19.
There is a low risk that he contracted the disease in Thailand, said Dr Tanarak Plipat, deputy director-general of the Disease Control Department.
“The investigation will seek the source of his infection, which is still a mystery since he was quarantined in Malaysia for 10 days,” he said.
He also mentioned the new “D614G” mutation of Covid-19 discovered in Malaysia, which is reportedly far more infectious than existing strains. He said Thailand’s screening process did not need to be changed to detect the mutation.