Recently, ternary AgSbS2 nanoparticles (NPs) have been demonstrated to be an eco-friendly potential solar absorber material. However, AgSbS2 has a band gap of Eg = 1.7 eV, larger than the Shockley-Queisser gap (1.2–1.4 eV) for optimal cell output. Here, Eg was tuned by substituting the anion S in AgSbS2 with Se via solution processing, forming AgSbSe2 NPs. The synthesized AgSbSe2 NPs exhibit the cubic crystal structure with an average size of 18 nm. The substitution reduced the Eg from 1.7 (AgSbS2) to 1.2 eV (AgSbSe2), leading to a significantly broader light absorption range. Solid-state AgSbSe2 quantum dot-sensitized solar cells were fabricated with Spiro-OMeTAD as hole transportation material. The best cell yielded a power conversion efficiency (PCE) of 1.01% under 1 sun. As the sun intensity was reduced, the PCE increased to 2.68% at 0.1 sun and further to 3.51% at 0.01 sun, a respectable PCE for a new solar material. The optimal Eg suggests that AgSbSe2 has potential to be a highly efficient solar absorber.