A Middle Triassic radiolarian fauna is present in the siliceous rock section at the Nan area along the Nan-Uttaradit suture zone, northern Thailand. The radiolarian fauna is composed of Triassocampe deweveri, T. nishimurai, Annulotriassocampe campanilis, Cenosphaera igoi, and other species and corresponds to the Anisian (Middle Triassic) Triassocampe deweveri fauna reported from Japan, Hungary, Italy, Austria, Far East of Russia, and northern and eastern Thailand. More than fifteen radiolarian species belonging to eight genera are investigated. The radiolarian-bearing rocks were probably deposited in a pelagic environment within the Nan-Uttaradit back-arc basin between the Simao and Indochina blocks during Middle Triassic time. This suggests that the Nan-Uttaradit back-arc basin, the Palaeo-Tethys ocean and Panthalassa ocean were probably connected by seaways at this time and might have shared the same oceanic circulation system. Furthermore, the complete closure of the Nan-Uttaradit back-arc basin between the Simao and the Indochina blocks is thought to have occurred at least after the Middle Triassic.