<p>This study investigates the efficacy of engineered cementitious composites (ECC) as a retrofitting material for concrete block masonry subjected to out-of-plane loading, through experimental and numerical analysis. The retrofitting approach involves applying a thin ECC layer (10–25 mm) to the tension side of masonry. Initially, small-scale tests were conducted to characterize the individual materials and their interfaces. Subsequently, a four-point bending test was performed on beam-like masonry specimens—five non-retrofitted and twelve retrofitted—to simulate out-of-plane wall behaviour. Experimental results revealed that ECC retrofitting significantly enhances masonry performance, increasing load-carrying capacity by up to 15.21 times and mid-span deflection by up to 31.64 times compared to non-retrofitted specimens. Retrofitted beams exhibited ductile and stable failure modes, contrasting the brittle failure of non-retrofitted beams. Numerical analysis complemented the experimental study using finite element modelling in ABAQUS, calibrated with experimental data. Calibrated models accurately predicted the load–deflection response, failure modes, and crack propagation patterns observed in tests. The load-carrying capacity and deflection improved proportionally with ECC thickness, with optimal results for beams retrofitted with 15–20 mm ECC layers. The proposed models demonstrated reliable performance in simulating out-of-plane behaviour, validating experimental conclusions. This study highlights ECC’s potential as an effective retrofitting material for masonry, providing significant improvements in load capacity, ductility, and stiffness. The findings emphasize the importance of ECC thickness and validate numerical modelling as a powerful tool for optimizing retrofitting strategies. Quantitative results, such as strength enhancement indices, provide a benchmark for future research and practical applications. </p>