Hepatitis B and C coinfection is commonly seen in clinical practice. In coinfected individuals, high levels of hepatitis C viremia are often associated with low levels of serum hepatitis B DNA. Hepatitis B reactivation in hepatitis C-infected patients treated with pegylated interferon and ribavirin has been reported, but severe or fulminant reactivation is uncommon. Hepatitis C treatment-associated hepatitis B reactivation in patients with chronic hepatitis C and isolated core antibody has not been reported previously. A 59-year-old white woman with chronic hepatitis C genotype 1B and isolated hepatitis B core antibody initiated treatment with simeprevir, sofosbuvir, and ribavirin for treatment of chronic hepatitis C. She responded very well to treatment initially with near normalization of aminotransferases and hepatitis C viral load suppressed to below the level of quantification after 4 weeks of treatment. At week 11 of a planned 12-week course, she developed fulminant hepatic failure due to hepatitis B reactivation and ultimately required liver transplantation. Fortunately, her posttransplant clinical course was unremarkable. This is the first report of hepatitis B reactivation in a patient with isolated hepatitis B core antibody leading to fulminant hepatic failure and liver transplantation after initiation of treatment with sofosbuvir, simeprevir, and ribavirin for hepatitis C. This case raises the concern for the risk of severe hepatitis B reactivation in hepatitis B and C-coinfected patients or chronic hepatitis C-infected patients with isolated hepatitis B core antibody treated with direct-acting antiviral drugs for hepatitis C.