Reinforced concrete deep beams (RCDBs) investigations are often complex because of the highly disturbed zones which may aggravate the shear performance under variable loadings. The shear capacity enhancement of RCDB using different aggregate sizes of 19 mm, 25 mm, and 50 mm has been investigated under three-point monotonic loading. Nine RCDBs with 750 × 170 × 225 mm dimensions were considered, and the beam was loaded at a 1.4 shear span to depth ratio. Three of the beams were designed without web reinforcement, and six were designed for web reinforcement with varied aggregate sizes. There was no significant difference in the shear strength of RCDBs considered however, a 50 mm aggregate beam was found capable of reducing the multiple crack propagations when compared to other aggregate-size beams. Additionally, the shear reinforcement increased the ductility and strength by over 30% and 20%, respectively. The applicability of 3-dimensional FEM extended to the investigation acceded with the shear response of the experiment exercising shear stiffening behavior. The estimated model from the modified ACI 318:05 can predict the shear capacity of the RCDB with higher accuracy. Since aggregate resists the load by aggregate interlock action, it is crucial to choose the right aggregate when building concrete structural components. The results of this study will help engineers choose the best aggregate for a certain structural element.