The vision of microchips that benefits from the high data rates of optical links has recently lead to novel concepts for the monolithic integration of active gain materials on silicon. In this letter, we use template-assisted selective epitaxy (TASE) for the monolithic integration of GaAs microsubstrates on silicon, which allows the subsequent integration of lattice-matched III-V laser cavities. We demonstrate the room-temperature operation of optically pumped microdisk-lasers with hexagonal top-facets and diameters as low as 1.2μm. The devices exhibit uniform emission spectra, low lasing thresholds of Pth=13.6 ± 0.7 pJ/pulse, and a high-temperature stability (T0=370 K). Scanning electron microscope and photoluminescence analysis of larger devices with a flat top-facet reveal a rotation of the crystal orientation and further reduce lasing thresholds. Our results show that the monolithic integration of III-V microsubstrates on silicon offers a versatile platform for on-chip light sources with excellent optical properties.