In this study, we describe a new occurrence of the well preserved freshwater bivalve Trigonioides (Trigonioides) quadratus from Lazha, Baingoin County, Tibet. It suggests an Early Cretaceous (Aptian—Albian, most likely Aptian) age for the fossil-yielding sediments, which can now be correlated to the Duoba Formation (equal to the lower part of the Langshan Formation) in Xainza. Our study suggests that this fossil-bearing volcano-sedimentary succession should be assigned to the Lower Cretaceous Zenong Group instead of the uppermost Cretaceous—Paleogene “Linzizong Group” as previously thought. We propose that the distribution of the Linzizong Group needs to be further investigated and the geological map of the Lhasa Block further refined. Our study provides some new information for understanding the spatial and temporal distribution of basins in the Lhasa Terrane, and highlights the need of more biostratigraphic studies in the region, which may provide critical constraints on the process and pattern of the northward subduction of the Neotethyan oceanic slab beneath South Asia since the onset of the Indo-Asian collision.