The upper Han River plays an important role in the Middle Route of the North to South Water Transfer Project in China. During an investigation of diatom biodiversity in this area, a new benthic monoraphid diatom, Achnanthidium ampliatum sp. nov. was observed from an alkaline tributary. This new species has relatively large valves, with a lanceolate axial area and distal raphe ends curved to the same side on the raphe valve, linear axial area in rapheless valves, with uniseriate striae and round to short slit-like areolae on both valves. We present light and scanning electron microscopic observations to document the new species. A. ampliatum sp. nov. is easily separated from other, similar Achnanthidium and Psammothidum taxa, and we provide comparisons with species that have similar outlines and valve features.