Oral injuries as a result of falling with a stick in the mouth are occasionally encountered in children. We herein report a pediatric case of trauma to the buccal mucosa caused by a toothbrush, resulting in herniation of the buccal fat pad. A 3-year-old boy fell off the sofa at home with a toothbrush in his mouth. His mother discovered him bleeding orally and brought him to the emergency unit of our hospital. The bleeding had ceased, and a reddish-yellow tissue mass was observed on the right buccal mucosa. The clinical diagnosis was a stab wound of the right buccal mucosa with hematoma and herniation of the buccal fat pad. Two days after the injury, the herniated buccal fat pad was excised under general anesthesia. The postoperative course was uneventful. The size of a mucosal stab wound by a toothbrush is larger than that caused by a chopstick, and may cause traumatic herniation of the buccal fat pad. Since its spontaneous loss is unexpected, reduction or excision of the fat pad may be necessary depending on the degree of herniation.