A parking assistance device includes a traveling route setting unit, a traveling route supply unit, an information acquiring unit, and a determination unit. The traveling route setting unit is configured to set a traveling route from a position of a vehicle to a parking spot in a parking lot. The traveling route supply unit is configured to supply information indicating the traveling route to the vehicle. The information acquiring unit is configured to acquire situation information containing the position of the vehicle and a situation of the parking lot. The determination unit is configured to determine whether the traveling route needs to be reset based on the situation information. When the determination unit determines that the traveling route needs to be reset, the traveling route setting unit resets the traveling route based on the situation information, and the traveling route supply unit supplies information indicating the reset traveling route.