An experimental investigation was performed to measure the regionally averaged heat transfer distributions and friction factors for fully developed turbulent flow in a rectangular channel with a width-to-height aspect ratio (AR = 3). Data for transverse 90-deg and 45-deg V-shaped, carbon steel ribs placed on one, two, or four sides of the channel were reported for a Reynolds number range (Re = 5,000–60,000). The pitch-to-rib height ratio () was set at 10 while the rib height-to-channel hydraulic diameter is 0.041. This corresponds to a roughness Reynolds number range For both rib configurations, results show the highest thermal performance was achieved in the channels with four-ribbed wall followed by the two and one-ribbed wall channels, respectively. Using the law of the wall similarity, friction roughness functions (R) and heat transfer roughness functions (G) were developed based on four-sided friction and heat transfer data. Moreover, friction and heat transfer data of similar rib designs were obtained from literature and correlated to the corresponding friction and heat transfer roughness functions. Covering a wide range of roughness Reynolds numbers, more robust heat transfer and friction correlations were developed which can be used by designers to accurately predict both heat transfer and friction in ribbed channels with different numbers of ribbed walls. Using these correlations, one can predict the measured friction and heat transfer data of this study with maximum errors of 9% and 7%, respectively.