The bedforms on a biofilm-coated sediment (biosediment) bed are different from those on a noncohesive sediment bed under an identical flow condition and sediment particle size without biofilm. Bedforms on biosediment beds can therefore modify the resistance to flow. In this study, experiments were conducted for the bedforms on both cohesive and biosediment beds. The biofilm coat around the fine cohesive sediment particles of a median size less than 0.1 mm was cultivated within a laboratory flume. The bedforms on biosediment beds are identified as dunes by comparing the Froude number and the Shields parameter. The equations of bedform steepness, bedform height, and equivalent roughness for biosediment beds are empirically derived. The maximum variations of equivalent roughness, Chézy coefficient, and Manning coefficient for a biosediment bed are 96.7, 60.5, and 44.3%, respectively, as compared to those for a noncohesive sediment bed under an identical flow condition and sediment size without biofilm. © 2017 American Society of Civil Engineers.