A battery temperature adjusting device for a vehicle on which a battery is mounted, the battery being a lithium ion battery disposed near a powertrain unit inside an engine bay, is provided. The device includes a first air duct provided to an intake passage configured to lead intake air to a combustion chamber of an engine, a second air duct provided to the intake passage and provided with an intake opening that opens toward a space between the powertrain unit and the battery, an intake-air-amount adjusting part, and a controller configured to acquire an ambient temperature of the powertrain unit. The controller increases a ratio of the second intake air amount relative to the sum of the first intake air amount and the second intake air amount, when the ambient temperature exceeds a first threshold temperature, compared with when the ambient temperature is below the first threshold temperature.