The rotational relaxation times of ammonium ion, tau (2r), in 11 solvents were determined by the measurements of the N-15 NMR spin-lattice relaxation times and the nuclear Overhauser enhancement (NOE) factors under the condition of negligible ion-ion interaction. The observed solvent dependence of the rotational relaxation times showed a poor correlation with those for predicted from a hydrodynamic (Stokes-Einstein-Debye) model or an electro hydrodynamic (Hubbard-Onsager-Felderhof) model, whereas a much better Linear relation was found for the plot of the logarithms of the observed rotational relaxation times versus Gutmann's solvent donor numbers. The results of these solvent dependence trends were compared with those perchlorate rotation, and the difference in the validity of the continuum models between the rotational motions of these two ions was discussed. A comparison was also made between the solvent dependences of the rotational and the translational diffusion of the ammonium ion.