A mononuclear tri-spin single-molecule magnet based on the rare earth radical [Tb(hfac)3(NITPhOEt)2] (NITPhOEt = 4'-ethoxy-phenyl-4,4,5,5-tetramethylimidazoline-1-oxyl-3-oxide) has been synthesized, structurally characterized and the alternating current signals show a slow relaxation of magnetization and frequency-dependent signals.