An extra-ovarian teratoma has been reported in a few cases of anti-N-methyl-D-aspartate receptor (NMDAR) encephalitis. A 25-year-old woman presented with psychiatric symptoms. We did not find an ovarian teratoma on initial examination, and her initial simple chest X-ray was normal. We incidentally found an abnormality on follow-up simple chest X-ray and diagnosed an anterior mediastinal teratoma. Therefore, in patients with suspected anti-NMDAR encephalitis, even if simple chest X-ray is normal, chest computed tomography should be performed to investigate a hidden teratoma.