3D printed polymeric reinforcement has been found able to improve the ductility of cementitious materials. However, due to the hydrophobic nature of commonly used 3D printing polymers, the bonding strength between the 3D printed polymers and cementitious matrix is extremely weak, which potentially hinders the mechanical performance of the reinforced composites. This work aims to improve the bonding properties by applying surface modifications on the 3D printed reinforcement, and eventually enhance the mechanical performance of the reinforced cementitious composites. Three types of surface coatings ingredients: epoxy resin (EP), sand sprinkled epoxy (SA) and short steel fibers sprinkled epoxy (SF) were used. Pull-out experiments are performed to study the bonding properties of the 3D printed reinforcement with different coatings. Then, uniaxial tensile and four-point-bending experiments are used to investigate the mechanical performance of the reinforced cementitious composites. A lattice type numerical model is applied to simulate the pull-out and tensile tests. The pull-out experiments indicate that the SA and SF reinforcement achieved approximately two times higher bonding strength than the uncoated and EP reinforcement. The tensile and flexural results suggest that the cementitious composites with SA and SF reinforcement achieved significantly better ductility (manifested by strain-hardening and deflection-hardening behavior) than the composites with uncoated and EP reinforcement. The numerical simulation results highly agree with the experimental findings, and further confirmed that the improved reinforcement-mortar bonding strength is the determinative factor that enhanced the composites mechanical performance. The findings of this work suggest that the sand and steel fiber surface coatings can effectively enhance the ductility of cementitious composites reinforced by 3D printed polymers.@en