An aqueous resin composition used to form an undercoat layer of a thermosensitive recording medium, comprising a water-repellent filler (A) having a water-repellent surface, which is composed of at least one of an organosilicon compound and an organofluorine compound, and an aqueous solution or water dispersion of a film-forming resin (B), which contains the water-repellent filler (A).