Baloxavir marboxil is an orally available prodrug of baloxavir acid. Japan was the first country to approve baloxavir marboxil as a treatment for influenza. The antiviral mechanism of action of baloxavir is unique; the drug blocks initiation of viral mRNA synthesis, thus preventing proliferation of the influenza virus. A single oral dose of baloxavir is usually well tolerated; it hastens alleviation of influenza symptoms and shortens the duration of viral shedding. However, novel influenza variants exhibiting over 10-fold reductions in baloxavir susceptibility emerged in baloxavir-treated patients. Although further clinical investigation is required to explore this issue, baloxavir may revolutionize our understanding of influenza virus biology.