Fossil records of Fagaceae have been widely reported in the Northern Hemisphere including many findings from East Asia. However, the history of their diversification within tropical Asia remains poorly understood, primarily constrained by the paucity of relevant fossil records from this region. In this study, we report well-preserved fossil leaves of Quercus section Cyclobalanopsis from the late Oligocene to Early Miocene of the Li Basin, northern Thailand. This new record comprises leaves with an acuminate apex, serrate margin (half-toothed in upper part of lamina), craspedodromous secondaries in the toothed part, opposite or alternate percurrent intercostal tertiary, anomocytic stomata and unicellular trichomes on abaxial epidermis. We assign these fossil leaves to a new species, Quercus liensis N. Thongsangtum, J. Huang et T. Su sp. nov., belonging to Quercus section Cyclobalanopsis . Detailed morphological comparisons indicate that the nearest living relative (NLR) of Q. liensis is most closely allied with Q. shennongii , a species currently distributed in the subtropical regions of southeastern China. In conjunction with previous palynological studies, this discovery suggests that a subtropical evergreen broad-leaved forest dominated by Fagaceae existed in northern Thailand during the late Oligocene – Early Miocene, which differs from the present-day tropical monsoon forest there, reflecting a significant change from warm-humid to modern seasonally hot-dry climate. We propose that the dramatic climate change and restructuring of vegetation composition were driven by several factors, including the Oligocene global cooling event and the tectonic southward extrusion of the Southeast Asian landmass, which imposed a long-term latitudinal shift.