Nitrogen-doped CaNb2O6 nanoplates with an ellipsoid-like morphology were prepared using the hydrothermal method, followed by heat treatment at various temperatures in an NH3 atmosphere. The prepared powders were characterized using X-ray diffraction, field-emission scanning electron microscopy, transmission electron microscopy, X-ray photoelectron spectroscopy, and UV-vis diffuse reflectance spectroscopy. It was found that the nitrogen doping in the CaNb2O6 nanoplates led to the formation of nanoparticles with a size of 10 nm at the surface and the red-shift of the light absorption edge into the visible light region. From the electronic band structure calculation using density functional theory (DFT), this was found to be caused by the broadening of the top of the valence band due to the hybridization of the O 2p and N 2p orbitals induced by the substitution of the nitrogen for the lattice oxygen. Compared to the un-doped powder, the N-doped CaNb2O6 nanoplate powder exhibited higher photocatalytic activities for the degradation of rhodamine B dye solution under visible light irradiation (> 420 nm). This higher photocatalytic activity was attributed to the higher optical absorption (smaller band gap) and higher surface area resulting from the nitridation process.