A self-priming pump assembly is disclosed. In an illustrative embodiment, the self-priming pump assembly includes a motor, a main pump having a main pump impeller, a vacuum pump having a vacuum pump impeller, and a drive shaft driven by the motor. The main pump impeller and the vacuum pump impeller may be directly coupled to and driven by the drive shaft, and may rotate about the rotation axis of the drive shaft. In some instances, the vacuum pump impeller is situated between the motor and the main pump impeller, while in other instances, the main pump impeller may be situated between the vacuum pump impeller and the motor. In some cases, bearings may be provided at or near the vacuum pump to help support the vacuum pump relative to the drive shaft.