In mainland Southeast Asia, Permian–Triassic successive subduction of the Paleo-Tethyan oceanic lithosphere formed an island-arc system called the Sukhothai Zone, along the margin of the Indochina Block. This zone has been considered to extend from Northern Thailand to southwestern Yunnan of China, but so far lacked concrete evidence from northern Laos in between. This makes basic geotectonic subdivision of mainland Southeast Asia ambiguous. In this study Triassic foraminifers fromlimestone distributed in the Long area of Luang Namtha Province (northwestern Laos) solve this problem. The relevant limestone has a kilometer-sized extent and was previously assigned to the Carboniferous–Early Permian based only on lithological grounds in regional geological mapping. Its Triassic age is demonstrated for the first time based on foraminifers. We discriminated from this limestone Aulotortus sinuosus, A. tumidus, Ophthalmidium danneri, Endotriada tyrrhenica, Endoteba obturata, Endotebanella kocaeliensis, Diplotremina astrofimbriata, Duostomina biconvexa, and Palaeolituonella majzoni, and concluded that the fauna is referable to the Carnian (early Late Triassic). Cement-rich reefal sediments consisting of Carniphytes multisiphonatus-microbial boundstone and sparse-allochem bioclastic grainstone containing abundant fragments of Plexoramea gracilis are common in this limestone. They suggest open-marine sedimentation on a shallowcarbonate platform, possibly comparable to a cement-dominated upper-slope “Tubiphytes” reef environment found elsewhere in the Triassic. In neighboring mainland Thailand, Carnian limestone with similar lithological and paleontological properties is found only in the Doi Long Formation distributed in the Sukhothai Zone. This evidence provides a firm basis for the tectonostratigraphic correlation of the Sukhothai Zone, in northern Laos. Consequently, our study is critical in giving a clue to clarify the distribution and extension in northern Laos of a Permian–Triassic island-arc domain, the Sukhothai Zone, which is one of themajor tectonostratigraphic units comprising mainland Southeast Asia. In this paper, we also described two encrusting foraminifers having multi-chambered (septate) shells, which have been seldom documented in Triassic foraminiferal literature.