The Immigration Bureau and the Ministry of Public Health are tracking about 783 African travellers who have entered Thailand since Nov 15.
They want to test them for the Omicron coronavirus variant.
The African visitors are believed to be in tourist destinations including Bangkok.
The immigration police and the ministry are also maximising Covid-19 control measures at the airports.
From Nov 15 until last Saturday, a total of 333 people from eight southern African countries, currently banned by several countries from entering, have entered Thailand.
The countries are Botswana, Eswatini, Lesotho, Malawi, Mozambique, Namibia, South Africa and Zimbabwe.
Thailand has also imposed a similar travel ban on visitors from these countries, starting on Wednesday.
Of the 333 people, three had already left Thailand. Twenty-three of them had been contacted through mobile phone texting and requested to take an RT-PCR test for Covid-19.
