Graphene is a promising material for producing optical devices because of its optical, electronic, thermal, and mechanical properties. Here, we demonstrated on-chip optical switches equipped with a graphene heater, which exhibited high modulation speed and efficiency. We designed the optimal structure of the optical switch with an add/drop-type racetrack resonator and two output waveguides (the through and drop ports) by the electromagnetic field calculation. We fabricated the optical switch in which the graphene microheater was directly placed on the resonator and directly observed its operation utilizing a near-infrared camera. As observed from the transmission spectra, this device exhibited high wavelength tuning efficiency of 0.24 nm/mW and high heating efficiency of 7.66 K·μm3/mW. Further, we measured the real-time high-speed operation at 100 kHz and verified that the graphene-based optical switch achieved high-speed modulation with 10%–90% rise and fall response times, 1.2 and 3.6 μs, respectively, thus confirming that they are significantly faster than typical optical switches that are based on racetrack resonators and metal heaters with response times of ∼100 μs. These graphene-based optical switches on silicon chips with high efficiency and speed are expected to enable high-performance silicon photonics and integrated optoelectronic applications.