A malignant, or immature, teratoma is an uncommon germ cell tumor. Teratomas are derived from embryonal germ layers (endoderm, mesoderm, ectoderm). Immature teratomas are partly distinguished from the mature form by the presence of neuroectoderm. Teratomas usually present in the ovaries or testes, retroperitoneum, mediastinum, or central nervous system. Immature teratomas most often occur in people under 20 years of age.

Signs of malignant teratoma depend on the location of the tumor and are usually secondary to mass effect or compression. These tumors may secrete alpha-fetoprotein, but this is not diagnostic.

Malignant teratomas are treated with surgery and chemotherapy. Malignant teratomas discovered at even an advanced stage typically have a good survival rate.

A malignant teratoma may occur years after the primary tumor.