The application of steel reinforced polymer (SRP) for external reinforcement of concrete structures is gaining attention, but its effectiveness is often hindered by premature debonding failure. To address this, a deeper understanding of the bond behavior between SRP and concrete is required, particularly considering the heterogeneity of concrete, which many prior studies have neglected. Size and distribution of coarse aggregates near the SRP-concrete interface can significantly influence bond capacity and alter failure mechanisms. This study employs a three-dimensional (3D) mesoscale lattice discrete particle model (LDPM) to analyze SRP strip-to-concrete bonded joints. The model, calibrated using results from well-documented experimental tests, simulates the SRP as a linear elastic material while employing a meso-scale representation of concrete that retains the information about the coarse aggregate distribution. The shear stress-slip law of the SRP-to-concrete interface is obtained by simulating single-lap shear (SLS) tests and three-point bending (TPB) tests of notched beams. The excellent agreement between numerical simulations and experimental data demonstrates the capacity of LDPM to realistically model the bond behavior and failure mechanisms of SRP-concrete joints. However, the numerical results highlight the differences between test methods in terms of obtained stress-slip laws. Therefore, the SLS test results cannot be directly utilized to analyze intermediate crack (IC) debonding failure in TPB tests, owing to the greater complexity of the latter’s failure mechanism.