Externally bonded fiber-reinforced polymer (FRP) composites represent a simple and economical solution for repairing and strengthening concrete structures. However, the potential for debonding failure in such strengthening systems becomes prominent when FRP-concrete bonds undergo environmental degradation induced by moisture. Low-viscosity Bisphenol A diglycidyl ether (DGEBA)-based epoxy adhesives are most commonly utilized in the engineering practice to bond wet-layup FRP to the concrete surface. This study evaluated the effects of the addition of commercial surface-modified nanosilica (SMNS), core-shell rubber (CSR) nanoparticles, and multi-walled carbon nanotubes (MWCNT) to neat epoxy on the mechanical properties of the adhesive, and strength and durability of FRP-concrete adhesively bonded joints. To determine the effects of environmental degradation, all specimens were subjected to the following environments: control - 23 degrees C at RH 50 +/- 10% for 18 weeks; and accelerated conditioning protocol (ACP) - water immersion at 45 +/- 1 degrees C for 18 weeks. Under control conditions, nanomodified epoxy exhibited enhanced mechanical properties compared to the neat epoxy. Following ACP, strength, elongation and modulus of elasticity of neat epoxy deteriorated significantly more than that of nanomodified adhesives. The bond strength of neat epoxy adhesive joints degraded most significantly (15%) following ACP, while nanomodified adhesive joints experienced minimal degradation of bond strength.