A vehicle seat air-conditioning device has a branch duct and a blower. The branch duct is branched laterally from a ventilation duct on a condition of being mounted in a vehicle. The ventilation duct delivers air, which is blown from an air conditioning unit that air-conditions an inside of a vehicle compartment, to a periphery of a rear seat. The blower draws air from the branch duct and blows the air into a ventilation path formed in a blowing target seat. The blow is arranged on a lateral side of the ventilation duct on the condition of being mounted in the vehicle. Since the blower is arranged on the lateral side of the ventilation duct in the vehicle, the blower can be located at a lower position regardless of a length of the ventilation duct in an upper-lower direction. Therefore, a space in the vehicle that is occupied in the upper-lower direction by the ventilation duct and the blower as a whole can be reduced.