Inversion-rotation transitions of the fully deuterated hydronium ion, D3O+, were observed for che first time by microwave spectroscopy. The ion was generated in a hollow cathode cell by de-glow discharge of a mixture of D2O and D-2. Twenty six P- and e-branch transitions were measured precisely for the lowest pair levels of inversion motion in the frequency region of 220-565 GHz. The ground-state inversion splitting and effective molecular constants for the upper and lower levels were determined by a least-squares fit of the measured line frequencies. The inversion splitting was determined to be 15.355 504 4(45) cm(-1), where the number in parentheses denotes three standard deviations of the fit.