Disclosed is a vehicle seat assembly with built-in heater having a configuration to minimize formation of creases. The vehicle seat assembly of the present invention comprises a seat heater. The seat heater is structured as a unit and comprised of a substrate and a heating wire. The heating wire has a first heater section and a second heater section. In the first heater section, a part of the segments bent on the left side extend leftward across the center. In the second heater section, a part of the segments bent on the right side extend rightward across the center. These segments are overlapped when viewed in the vertical direction. This configuration minimizes formation of creases in the middle area.