In this paper, an experimental and numerical study was presented for strengthened composite steel–concrete beams using externally post-tensioning tendons with a partial shear connection. Four composite steel–concrete beams under a static monotonic load were tested to validate the numerical model and investigate the different failure mechanisms. The cracking behaviors of the composite beams and overall moment of resistance have been experimentally and numerically studied. The validated numerical model was used to perform a parametric study. It was found that the initial tendon force increased by 18–23% due to the beam bending, which led to an enhancement in the performance of the beam. Moreover, increasing the degree of shear connection (DOSC) from 40% to 100% led to an increase in the mean maximum load by 46% and to decrease the mean maximum deflection by 22.5%. In addition, there was an improvement in the slippage by achieving 28% decrease. Also, there was a decrease in steel flange micro-strain by about 51%.