The combined effects of system rotation and rib-roughness on turbulent channel flow have been investigated by means of direct numerical simulations. The 40 wall-units high square ribs were placed on both walls in a non-staggered arrangement with pitch-to-height ratio 8. Mean flow fields and turbulence statistics for rotation numbers Ro= 2 and 6 were presented and compared with corresponding data for Ro= 0. The flow field in the vicinity of the ribs was affected differently on the two sides of the rotating channel. The separated flow region behind the pressure-side ribs shrinked with increasing Ro and the originally d-type roughness turned into a k-type roughness. At Ro= 6, a pressure-loss reduction of about 20% was found. In spite of the 10% blockage due to the ribs, the flow field exhibited a statistical homogeneity in the streamwise direction over more than half of the channel. The turbulence statistics were substantially affected by the system rotation with enhanced and reduced turbulence levels along the pressure and suction sides, respectively. Among other things, we also learned that the combined influences of rib-roughness and system rotation on the turbulent flow field in rotating channels cannot be foreseen by straightforward superposition. © 2014 Elsevier Inc.