The formation of large impact basins (diameter D ≥ 300 km) was an important process in the early geological evolution of Mercury and influenced the planet's topography, stratigraphy, and crustal structure. We catalog and characterize this basin population on Mercury from global observations by the MESSENGER spacecraft, and we use the new data to evaluate basins suggested on the basis of the Mariner 10 flybys. Forty-six certain or probable impact basins are recognized; a few additional basins that may have been degraded to the point of ambiguity are plausible on the basis of new data but are classified as uncertain. The spatial density of large basins (D ≥ 500 km) on Mercury is lower than that on the Moon. Morphological characteristics of basins on Mercury suggest that on average they are more degraded than lunar basins. These observations are consistent with more efficient modification, degradation, and obliteration of the largest basins on Mercury than on the Moon. This distinction may be a result of differences in the basin formation process (producing fewer rings), relaxation of topography after basin formation (subduing relief), or rates of volcanism (burying basin rings and interiors) during the period of heavy bombardment on Mercury from those on the Moon.