We report a case of anti-N-methyl-D-aspartate-receptor (anti-NMDAR) encephalitis associated with an immature teratoma developed in a 38-year-old woman. Positron emission tomography (PET) revealed focal intense uptake of 18F-fluorodeoxyglucose in an area of the brain corresponding to the right medial temporal lobe as well as an intrapelvic tumor. After the PET examination, the patient complained of disorientation and short-term memory loss. The ovarian tumor was resected and diagnosed as an immature teratoma. The cerebrospinal fluid analysis was positive for anti-NMDAR antibody. After surgery, the patient’s neurologic symptoms improved. The PET finding of encephalitis associated with an immature teratoma was unexpected.