Thrombocytopenia during pregnancy has many different causes, but Mycoplasma pneumoniae is not usually considered one of the several pathogens that induce thrombocytopenia. Herein, we present a case of severe thrombocytopenia that was associated with M. pneumoniae during pregnancy. The patient experienced fever, cough, and cytopenia with M. pneumoniae-specific IgM antibody increasing from 40-fold to 160-fold during the 2 weeks of illness. A diagnosis was made after excluding other diseases that cause thrombocytopenia. The patient was successfully treated with azithromycin hydrate, and she delivered a healthy newborn without any complications. Pregnant women who are infected with M. pneumoniae during pregnancy may develop severe and fatal thrombocytopenia. Prompt diagnosis and initiation of treatment lead to early recovery.