This study focuses on the dielectric properties of 21.9-nm spherical zinc oxide (ZnO) nanoparticles (NPs) at room temperature, as a dry powder and suspended in a liquid. Impedance spectra in the frequency range of 100 Hz to 5.1 MHz were used to investigate the frequency-dependent dielectric properties of ZnO NPs. The commercially available ZnO NPs used in this study were suspended in variable volume fractions up to ~1% in deionized (DI) water and unrefined organic coconut oil and subjected to three sonication conditions: no sonication (NS), 1 hour of bath sonication (BS), and 1 hour of bath sonication followed by probe sonication throughout the experiment (CS, “concurrent sonication”) to determine sonication dependence. Small volumes of the resulting suspension were injected sequentially into a dielectric cell for measuring frequency response. Dry particle tests were conducted similarly. Impedance data suggests that the dielectric behavior of ZnO NPs in a liquid suspension is highly dependent on sonication before and during the test and exhibited a strong dependence of dipole with the polarity of the liquid at low frequencies. In addition, a higher dielectric constant of ZnO NPs was observed when the nanoparticles were in suspension than as a dry powder. For frequencies between 100 Hz and 100 kHz, the average dielectric constant of ZnO NPs in DI water, in unrefined coconut oil, and as a dry particle are 368.63, 24.43, and 7.25, respectively.