Psychogenic polydipsia (PP) is a clinical condition characterized by excessive fluid intake in the absence of physiological stimuli to drink. The etiology of compulsive water drinking is not known yet. It is common in people with chronic mental illness, especially in schizophrenia. Obsessive-compulsive disorder (OCD) is characterized by the presence of recurrent obsessions and compulsions which may cause anxiety in the person. The age of onset is bimodal, before puberty and early adulthood. In this case report, we will discuss the PP situation after the start of sertraline treatment in a case of 12-year-old girl who started sertraline treatment with the diagnosis of OCD.