Magnetic resonance imaging (MRI) allows for body composition assessment, including determination of fat-free muscle area (FFMA). This study aimed to evaluate the prognostic value of FFMA, opportunistically measured from pre-interventional liver MRI, to predict outcome in patients receiving radioembolization (RE) for treatment of hepatocellular carcinoma (HCC). Patients with high FFMA showed significantly increased overall survival following treatment. On multivariate analysis, FFMA was an independent predictor of survival and thereby may reveal additional prognostic information in these patients. FFMA may become a promising new biomarker, since it is easy to assess from routine clinical imaging and represents a potential therapeutic target.