A vehicle electricity supply control system, including: an acquisition section configured to acquire a power consumption of an auxiliary system, the auxiliary system being supplied with electricity from an auxiliary battery; and a control section configured to, in a first supply mode in which electricity is supplied to a drive battery and the auxiliary system and in a case in which the power consumption of the auxiliary system is greater than a predetermined value, control a ratio of electricity supplied to the drive battery and the auxiliary system such that an electricity supply amount to the auxiliary system becomes greater than in a case in which the power consumption of the auxiliary system is less than the predetermined value.