In principle, the spin-1/2 plutonium-239 (Pu-239) nucleus should be active in nuclear magnetic resonance spectroscopy. However, its signal has eluded detection for the past 50 years. Here, we report observation of a Pu-239 resonance from a solid sample of plutonium dioxide (PuO2) subjected to a wide scan of external magnetic field values (3 to 8 tesla) at a temperature of 4 kelvin. By mapping the external field dependence of the measured resonance frequency, we determined the nuclear gyromagnetic ratio (239)gamma n(PuO2)/2 pi to be 2.856 +/- 0.001 megahertz per tesla (MHz/T). Assuming a free-ion value for the Pu4+ hyperfine coupling constant, we estimated a bare (239)gamma n/2 pi value of similar to 2.29 MHz/T, corresponding to a nuclear magnetic moment of mu(n) approximate to 0.15 mu(N) (where mu(N) is the nuclear magneton).