Objectives To compare the presence of washout and the diagnostic performance of computed tomography (CT) and magnetic resonance imaging (MRI) for hepatocellular carcinoma (HCC) according to the presence of hepatic steatosis. Methods This retrospective study included 566 patients with chronic liver disease who had undergone hepatic resection for hepatic tumors (482 HCCs and 84 non-HCCs) between January 2016 and June 2018 and had available multiphasic CT and MR images. Patients were allocated in the fatty liver ( n  = 141) or non-fatty liver ( n  = 425) group according to the presence of hepatic steatosis, defined as lipid droplets in at least 5% of hepatocytes on pathological examination. The presence of HCC washout and the diagnostic performance of CT and MRI for HCC were compared between the groups. Results HCC washout was less frequently seen in the fatty liver group than in the non-fatty liver group on CT (61.5% vs. 88.9%, p  < 0.001), whereas it was similarly present on MRI in both groups (77.0% vs. 74.4%, p  = 0.565). For diagnosis of HCC, the sensitivity (53.3% vs. 80.0%, p  < 0.001) and accuracy (53.9% vs. 80.9%, p  < 0.001) of CT were lower in the fatty liver group than in the non-fatty liver group. However, for MRI, these values were not significantly different between the groups ( p  > 0.05). Conclusions Hepatic steatosis significantly decreased the performance of CT for the diagnosis of HCC, whereas it did not significantly alter the performance of MRI. Key Points • Unlike MRI, there is vanishing HCC washout on CT caused by the background hepatic steatosis . • The diagnostic performance of CT for the diagnosis of HCC was significantly altered by hepatic steatosis . • The optimal cutoff HU value of the liver parenchyma for the vanishing washout of HCC was < 50 HU on unenhanced CT images .