The orientation of fibers in the cardiac wall has been shown to strongly affect the mechanical behavior of the heart. Experimental studies indicate that fiber orientation adapts in response to changes in tissue mechanical properties. Previous computational studies considered a model of strain-based fiber reorientation, applied in a generic ellipsoidal left ventricular geometry. In this study, we investigate how predicted fiber orientation changes when using an anatomically realistic left ventricular geometry, obtained from the Cardiac Atlas Project. Results show that, through adaptation, fibers became more circumferentially oriented at the endocardium and more longitudinally oriented at the epicardium in both an ellipsoidal and realistic geometry. In addition, fiber reorientation enhanced pump function and improved the spatial homogeneity of local fiber mechanics in both geometries. As adaptive fiber reorientation progressed, there was a tendency toward divergence in the ellipsoidal geometry, which was not yet observed in the more realistic geometry. The reorientation model might be used to complement models with a patient-specific geometry with a dedicated fiber field, instead of using a generic rule-based fiber field.