In this paper, we report a catalyst-free topochemical method, combined with molten salt synthesis (MSS), to synthesize, on a large scale, rodlike and platelet single crystals of Nb2O5. Rodlike KNb3O8 and platelet K4Nb6O17, which were fabricated as the precursors by the molten salt method, were treated by proton exchange and heat treatment to synthesize the rodlike H-Nb2O5 and platelet T-Nb2O5 single crystal, respectively. The synthesized niobium pentaoxides retained the rodlike and platelet shapes of their precursors. The structural changes involved in the process were investigated by Raman spectroscopy, X-ray diffraction, scanning electron microscopy, and transmission electron microscopy. A possible topochemical reaction mechanism is proposed. Furthermore, rodlike and platelet KNbO3 powders were derived from stable H-Nb2O5 and T-Nb2O5, respectively.