The aim of this paper is to study the effect of using Ground Granulated Blast Furnace Slag (GGBS) as a partial replacement to cement in reinforced concrete (RC) beams. A total of eight beams were cast with different percentages of GGBS replacement of 0%, 50%, 70%, and 90%, respectively. The performance of the tested specimens were evaluated and compared to that of a control beam without GGBS (0%). In addition, the concrete compressive and tensile strength of the different concrete mixes were evaluated and compared. Overall, test results indicated that the compressive and tensile strength of the different mixtures were quite similar. In addition, the performance of RC beams with GGBS replacement up to 70% is similar to that without GGBS. However, the stiffness and strength for the beam specimens with 90% GGBS were lower than that without GGBS by 16% and 6%, respectively. It was also concluded that the use of high percentage of GGBS up to 70% as a replacement to cement is practical and will not comprise the performance of RC beams. Furthermore, such replacement will contribute to the reduction in CO2 emission (carbon footprint) and therefore encourage the use of such sustainable and green concrete.