Simultaneously enhancing the response and humidity resistance of p-type metal oxide semiconductor (MOS) is of vital importance for the development of efficient sensor technology. In this work, we successfully synthesized Co3O4 hierarchical structure composed of nanosheets doped with a rare earth element (Sm) through a facile solvothermal strategy, intending to increase the response and humidity resistance toward CO. The results show that the sensor based on 5 mol% Sm–Co3O4 has a response value of 2.6–100 ppm CO at 210 °C, which is approximately 2.4 times higher than that of the pristine Co3O4. Simultaneously, the 5 mol% Sm–Co3O4 sensor exhibits a lower response change rate [(Sdry-Swet)/Sdry = 31.7 %] than that of the bare Co3O4 (43.2 %) from 32 % RH to 83 % RH. Besides, the sensor also presents a low detection limit (500 ppb), a short response time (20 s), and good selectivity. During the 10-cycle testing process, the 5 % Sm–Co3O4 sensor shows minimal variation in resistance, indicating good response reproducibility. Furthermore, during a 30-day long-term stability test, the sensor maintains a stable response of approximately 2.6 toward 100 ppm CO, with the baseline resistance remaining around 1.3 kΩ, confirming its outstanding long-term stability. The enhanced CO-sensing properties are owing to the abundant oxygen vacancies caused by Sm doping, increased specific surface area (81.1 m2 g−1), and the Co3+/Co2+ ratio. This work provides a facile and novel strategy for enhancing the response and humidity resistance of p-type oxide semiconductors.