Spinel ZnGa2O4 ceramics were synthesized by conventional solid-state method and their microwave dielectric properties were investigated. The phase evolution and microstructures of specimens were studied by XRD and SEM. The textured surface microstructures of ZnGa2O4 ceramics formed at high sintering temperatures. The spinel-structured ZnGa2O4 ceramics sintered at 1385°C exhibited excellent microwave dielectric properties: a dielectric constant (εr) of 10.4, a quality factor (Q × f) of 94.600 GHz, and a temperature coefficient of resonant frequency (τf) of −27 ppm/°C. ZnGa2O4 ceramics have a low sintering temperature, a wide temperature region, and a small negative τf value. They are promising candidate materials for millimeter-wave devices.