Heavily co-doped Cu2SnS3 can achieve a high power factor by relying on a high electrical conductivity (σ), which subsequently limits the ZT value with a large electronic thermal conductivity (κe). We report here an enhanced ZT for Cu2Sn0.9Co0.1S3 decorated with micro-nanoscale AgSnSe2 along grain boundaries. The AgSnSe2 phase served as a charge carrier filter by ionized impurity scattering, with a noticeable bottoming out of carrier mobility and a rapid increase in the Seebeck coefficient as the temperature increased from 423 to 573 K, which properly reduced the large σ and κe while maintaining a high power factor of approximately 10 μW cm-1 K-2 at 773 K. Lattice thermal conductivity was markedly suppressed, and a low total thermal conductivity was obtained with strengthened phonon scattering by the AgSnSe2 phase as a phonon barrier. With the synergistic effects on electrical and thermal transport, a maximum ZT of 0.93 at 773 K was achieved in Cu2Sn0.9Co0.1S3-3 wt% AgSnSe2.