A power supply device mounting structure for a power supply device disposed below a seat disposed in a vehicle interior is provided. The power supply device mounting structure includes a plate member and a contact portion. The plate member is disposed between the power supply device and a seat surface portion of the seat and the plate member covers an upper surface of the power supply device. The contact portion is provided at a position facing an edge portion of the plate member and the contact portion comes into contact with the seat surface portion when a load is input from the seat surface portion.