The photoluminescence of AgIn5S8 nanoparticles was examined to clarify the emissive relaxation processes of defect states and to explain the highly efficient photoluminescence of defect states. The large Stokes shift of the defect emission was explained by strong electron-phonon coupling in the nanoparticles. Steady-state and time-resolved photoluminescence spectroscopy indicated two emissive defect states with characteristic emission energies and lifetimes. Change of the surface-to-volume ratio in the nanoparticles affected the relative contribution of the two states, implying that defect emission in higher energy was attributable to surface-related defects. The defect emission in lower energy was attributable to intrinsic defects, which were also present in bulk. The quantum yield of the surface defects was larger than that of the intrinsic defects, which accounted for the unusually high quantum yield of AgIn5S8 nanoparticles, although the origin of emission was the defect states, not the exciton recombination found in typical semiconductor nanoparticles.