Phylloid algal reef mounds were commonly developed on Pennsylvanian—Permian carbonate platforms and today form globally distributed hydrocarbon reservoirs. The Pennsylvanian—Permian Pha Nok Khao (PNK) carbonate platform in the Loei-Phetchabun Foldbelt provides a direct analogue for gas-producing subsurface platforms beneath the nearby Khorat Basin. Although phylloid algal reef mounds are recognised across the PNK Platform, to date only patch reefs and isolated mounds have been described. This study describes two sections representing an ~80-m-thick succession of closely spaced phylloid reef mounds and intermound facies. Reef mounds are interbedded with deep to shallow subtidal lagoon and tidal flat facies and are interpreted to have formed on the inner platform. The phylloid reef mounds follow the build-and-fill style of growth, in which relief-building and relief-filling represent separate stages. Between mounds, lagoonal sediments fill topographic lows. Subsequent reef mounds were not restricted to building on existing mounds, but distributed within the inner platform. Thick peritidal facies overlie the mounds suggesting development of a shallow carbonate platform in this region during overall regressive conditions in the Late Pennsylvanian (Moscovian—Kasimovian).