Reductive amination generates an important reaction in fine chemical synthesis. The employment of water as the hydrogen donor for reductive amination might solve the long-time hydrogen supply problem. Here, we present our new results on reductive N-methylation reactions of amine with paraformaldehyde with water as the co-hydrogen donor catalyzed by a simple supported nanogold catalyst, i.e., Au/Al₂O₃. N-Methyl amines or N,N-dimethyl amines can be selectively synthesized with excellent yields. Isotope tracing reactions confirmed the transformation of hydrogen from water in the final product. In addition, this method can be applied in the N-methylation reactions of bioactive molecules with excellent performance. This concept may supply a potential methodology for sustainable reductive amination.