This paper presents a numerical study investigating the effect of ribbed pinfins on the thermal–hydraulic performance of a microchannel heat sink. A three-dimensional numerical model of a single rectangular microchannel with rectangular pinfins having ribs on them is developed, and simulations are performed for different rib configurations under laminar flow conditions with deionized water as the coolant. The study is conducted in two parts. The first part examines the influence of varying numbers of ribs on the pinfins while maintaining a constant rib width. The results show that increasing the number of ribs initially enhances heat transfer, with the case having two ribs (R2) exhibiting the highest Nusselt number ratio and lowest thermal resistance, up to 16 % lower than the nonribbed case (R0). However, a further increase in the number of ribs leads to a decrease in heat transfer performance due to the reduced cross-sectional area for heat conduction through the ribs. Moreover, the highest performance factor is exhibited by the case consisting of seven ribs (R7). The second part explores the impact of varying rib width, using the optimal R7 configuration based on the performance factor from the first part. The results indicate that increasing the rib width significantly improves heat transfer, with the widest rib case (W7) achieving up to a 25 % increase in Nusselt number compared to a plain microchannel. The study provides insights into the design of efficient microchannel heat sinks with ribbed pinfins for effective cooling of high-heat-flux electronic devices.