The relationship between prolactinoma and obesity has not been well explained yet. This report introduced the diagnosis and treatment course of a severely obese male patient. After a series of examinations, the patient was finally diagnosed as prolactinoma after excluding other endocrine-related diseases, like diabetes. His plasma prolactin level (PRL) dropped after treatment with bromocriptine, and so did his weight. It is indicated that prolactinoma may lead to the occurrence of obesity, especially in male patients. The abnormal insulin metabolism caused by the elevation of PRL may be the main reason for this phenomenon. The case provides a fresh idea to control the weight of obese teenagers.