We demonstrated, for the first time, formation of the Na-alpha'-GeGaON (NamGe12-(m+n)Gam+nOnN16-n) solid solution, an analogue of the well-established alpha'-SiAlON system. We successfully synthesized single-phase powder samples by reduction nitridation of Na2CO3-GeO2-Ga2O3, in the solubility range of m approximate to 0.8-1.7 with n approximate to 0.2-0.3. The Rietveld refinement of powder X-ray diffraction data for Na-alpha'-GeGaON was conducted on the basis of the space group P31c of alpha'-SiAlON, and the refinement converged with R-B = 1.78% and R-F = 1.02% for the composition of Na1.26(1)Ge10.50-Ga1.5O0.24(1)N15.76(1), indicating reliably the isomorphism between the SiAlON and GeGaON systems. The results of Na-23 solid-state nuclear magnetic resonance (NMR) spectroscopy clearly showed a single peak at the chemical shift of similar to 16 ppm, further proving the accommodation of Na in the alpha'-GeGaON matrix with the expected coordination environment. The synthesized Na-alpha'-GeGaON exhibited stable photocatalytic activity for the evolution of H-2 from water under ultraviolet irradiation, which was comparable to that attained by beta-Ge3N4.