Corticosteroid treatment sometimes causes psychiatric side effects such as mania, depression, and psychosis. It is believed that exogenous corticosteroids lead to dysregulation of corticosteroid signaling and neurotransmitters in a dose-dependent manner. Therefore, the administration of corticosteroids is at risk of worsening bipolar disorder. Here, we present a case of a female patient with subthreshold untreated bipolar disorder who experienced a severe manic episode after corticosteroid treatment. She had a history of childhood asthma but no remarkable psychiatric medical history except for subthreshold mood swings. At age 30, she had her first asthma attack in over 10 years, for which she received intravenous corticosteroids. A week later she was admitted to a psychiatric hospital due to severe manic symptoms. She was diagnosed with corticosteroid-induced mania and was treated with olanzapine. After remission, olanzapine was gradually reduced. However, she experienced a recurrent manic episode. Her diagnosis was updated to bipolar disorder, and she has resumed medication. This case highlights the risk of corticosteroids worsening bipolar disorder and the need to carefully assess previous psychiatric symptoms before using corticosteroids.