We report the synthesis, crystal structure, thermal, dielectric, IR, and Raman studies of [NH4] [Mg(HCOO)(3)] formate. Single-crystal X-ray diffraction shows that it crystallizes in the hexagonal space group P6(3)22, with orientationally disordered NH4+ ions located in the cages of the network. Upon cooling, [NH4][Mg(HCOO)(3)] undergoes a phase transition at around 255 K to the ferroelectric P6(3) structure. Raman and IR spectra show a strong increase in intensity of the N-H stretching bands as well as narrowing of the bands related to the NH4+ ions upon cooling. These changes indicate that the phase transition is due to orientational ordering of the NH4+ ions. Analysis of the Raman data show, however, that the rotational and translational motions of NH4+ do not freeze completely at the phase transition but exhibit further slowing down below 255 K, and the motional freezing becomes nearly complete below 140 K.