Pimples on the roof of the mouth can be caused by acne, folliculitis and herpes simplex. 1. Acne: Pimples on the roof of the mouth may occur if there is an endocrine imbalance caused by an irregular diet, sleep problems, or prolonged late nights, which promotes the secretion of sebum. 2. Folliculitis: Inflammatory reaction of the hair follicles of the skin around the mouth due to infection with bacteria and other pathogens can also cause pimples around the mouth, accompanied by redness, swelling, pain and other symptoms. 3. Herpes simplex: pimples on the upper side of the mouth may also be herpes simplex, mostly due to the patient’s body immunity decline, caused by infection with the herpes simplex virus. If you have pimples on your mouth, it is recommended that you go to a regular hospital to get a clear diagnosis and timely treatment to avoid delaying your condition.