Two carbonate successions constituting Paleozoic–Mesozoic basement rocks distributed in the Nakhon Sawan–Uthai Thani area in the Chao Phraya Central Plain of Thailand were investigated to understand basic geotectonic subdivision of the area. Of them, we newly proposed the term Khao Pathawi Limestone for a Triassic succession exposed in eastern Uthai Thani Province, from which a rich foraminiferal fauna consisting of Aulotortus sinuosus, A. friedli, Tetrataxis inflata, Diplotermina subangulata, Agathammina austroalpina, Ammobaculites rhaeticus, Alpinophragmium perforatum, Gaudryina triadica, and others was discriminated. They clearly suggest a Late Triassic age and highly probably Norian/Rhaetian. This limestone succession is considered to have formed a carbonate platform and is comparable to those on the Triassic Lampang–Phrae Basin in the Sukhothai Zone of Northern Thailand. In contrast, the Uthai Thani Limestone exposed just west of the Khao Pathawi Limestone, in the form of NNW–SSE trending monadnocks, represents a slightly metamorphosed, thick-bedded Permian succession. Its outcrop alignment is distinct from the north–south trending Khao Pathawi Limestone, suggesting their structural separation. The Uthai Thani Limestone is correlated to the Sai Yok Formation and the Ratburi Limestone, both of which are well-known as typical Permian successions on the Sibumasu Block. By integrating these data from the carbonates, together with available geological information from other Paleozoic–Mesozoic basement units in the Central Plain, such as petrochemical properties of granitoids and volcanic/volcaniclastic rocks and litho- and chronostratigraphic characteristics of sedimentary successions, including the Permian Khao Gob Chert, we explain that the central part of the Nakhon Sawan–Uthai Thani area where the Khao Pathawi Limestone and the Khao Gob Chert are distributed, corresponds geotectonically to the direct southern extension of the Sukhothai Zone of Northern Thailand. Thus, as is the case of Northern Thailand, we recognized three geotectonic domains also in the Central Plain; they are the Indochina Block (part of Cathaysialand), Sukhothai Zone (Permian–Triassic island arc system), and Sibumasu Block (part of Gondwanaland) from east to west. To the south, the Sukhothai Zone probably extends to the Klaeng Zone of Southeast Thailand. In this paper, Endotriada martiniae Ueno, Miyahigashi and Charoentitirat, n. sp. from the Khao Pathawi Limestone is also described.