A ultrathin HNb₃O₈ nanosheet was successfully prepared through a top-down approach. Atomic force microscopy (AFM) further confirmed that the thickness of the nanosheet was about 1.30 nm. The structure and the surface chemical state of the HNb₃O₈ nanosheet were well-characterized using X-ray diffraction (XRD), high-resolution transmission electron microscopy (HRTEM), X-ray absorption fine structure (XAFS), Raman and X-ray photoelectron spectroscopy (XPS). The photocatalytic hydrogen evolution activity of the HNb₃O₈ nanosheet was about 4 times higher than that of layered HNb₃O₈ under ultraviolet irradiation. The enhanced activity was ascribed to the unique two-dimensional structure with a molecular thickness that leads to the effective separation of the photogenerated carriers. Moreover, a considerable variation in the photocatalytic hydrogen evolution activity of the HNb₃O₈ nanosheet was observed when suitable metals were loaded on the nanosheet via in situ photodeposition.