The new subspecies were found among more than 2,000 new cases sampled randomly from around the country, said Department of Medical Sciences chief Dr Supakit Sirilak.
The AY.4 (B.1.617.2.4) subspecies was found in Pathum Thani (4 cases), Buri Ram (1 case), Kamphaeng Phet (1 case), Chiang Mai (1 case), Samut Prakan (1 case), and Chonburi (1 case).
The AY.6 (B.1.617.2.6) subspecies was found in Bangkok (1 case).
The AY.10 (B.1.617.2.10) subspecies was found in Bangkok (1 case).
And the AY.12 (B.1.617.2.15) subspecies was found in Bangkok’s Phaya Thai district (1 case) and Surat Thani (2 cases).
All four subspecies have been recorded previously in other countries, including Britain, Spain, Denmark and the United States.
Dr Wasun Chantratita, head of Mahidol University's Centre for Medical Genomics, said the new subspecies underlined the need for prevention via disease controls and vaccination.
Thailand logged 17,165 new Covid-19 cases on Tuesday, the fewest since July 30, with a death toll of 226.