We report that Ga-doped and Ga-In-codoped n-type PbS samples show excellent thermoelectric performance in the intermediate temperature range. First-principles electronic structure calculations reveal that Ga doping can cause Fermi level pinning in PbS by introducing a gap state between the conduction and valence bands. Furthermore, Ga-In codoping introduces an extra conduction band. These added electronic features lead to high electron mobilities up to mu(H) similar to 630 cm(2) V-1 s(-1) for n of 1.67 X 10(19) cm(-3) and significantly enhanced Seebeck coefficients in PbS. Consequently, we obtained a maximum power factor of similar to 32 mu W cm(-1) K-2 at 300 K for Pb0.9875Ga0.0125S, which is the highest reported for PbS-based systems giving a room-temperature figure of merit, ZT, of similar to 0.35 and similar to 0.82 at 923 K. For the codoped Pb0.9865Ga0.0125In0.001S, the maximum ZT rises to similar to 1.0 at 923 K and achieves a record-high average ZT (ZT(avg)) of similar to 0.74 in the temperature range of 400-923 K.