This study investigated the effects of the addition of Nb2O5 and sintering temperature on the properties of Bi2Mo2O9 ceramics. The ceramics were sintered in air at temperatures ranging from 620°C to 680°C. The addition of small amounts of Nb2O5 as a dopant significantly affected the crystalline phase and the microwave dielectric properties of the Bi2Mo2O9 ceramics. The secondary phase, γ-Bi2MoO6, was observed when Nb2O5 was added. However, unlike the Bi2Mo2O9 ceramic without Nb2O5 sintered above 645°C, the ceramics with 3 mol% Nb2O5 contained no γ-Bi2MoO6 when sintered at 660°C. The Q × f value and τf of the Bi2Mo2O9 ceramics were improved by Nb2O5 doping. The Bi2Mo2O9 ceramics doped with 2 mol% Nb2O5 exhibited the best microwave dielectric properties, with a permittivity of 36.5, a Q × f value (f = resonant frequency, Q = 1/dielectric loss at f) of 14100 GHz and τf of +5.5 ppm/°C after sintering at 620°C.