Single-crystalline wurtzite InGaAs/InGaP nanopillars directly grown on a lattice-mismatched silicon substrate are demonstrated. The nanopillar growth is in a core-shell manner and gives a sharp, defect-free heterostructure interface. The InGaP shell provides excellent surface passivation effect for InGaAs nanopillars, as attested by 50-times stronger photoluminescence intensities and 5-times greater enhancements in the carrier recombination lifetimes, compared to the unpassivated ones. A record value of 16.8% internal quantum efficiency for InGaAs-based nanopillars was attained with a 50-nm-thick InGaP passivation layer. A room-temperature optically pumped laser was achieved from single, as-grown InGaAs nanopillars on silicon with a record-low threshold. Superior material qualities of these InGaP-passivated InGaAs nanopillars indicate the possibility of realizing high-performance optoelectronic devices for photovoltaics, optical communication, semiconductor nanophotonics, and heterogeneous integration of III-V materials on silicon. © 2014 American Chemical Society.