Hybrid octagonal-ring microlasers are investigated for realizing a stable output from a silicon waveguide based on a two-dimensional simulation. The inner radius of the ring is optimized to achieve single-mode and low-threshold operation. Using the divinylsiloxane-benzocyclobutene (DVS-BCB) bonding technique, a hybrid AlGaInAs/Si octagonal-ring microlaser vertically coupled to a silicon waveguide is fabricated with a side length of 21.6 μm and an inner radius of 15 μm. A single transverse-mode operation is achieved with a threshold current density of 0.8 kA/cm2 and a side-mode suppression ratio above 30 dB, and a stable output from the lower silicon waveguide is obtained.