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Hybrid material systems are a promising approach for extending the capabilities of silicon photonics. Given the weak electro‐optic and thermo‐optic effects in silicon, there is intense interest in integrating an ultrafast‐switching phase‐change material with a large refractive index contrast into the waveguide, such as vanadium dioxide (VO₂). It is well established that the phase transition in VO₂thin films can be triggered by ultrafast, 800 nm laser pulses, and that pump‐laser fluence is a critical determinant of the recovery time of thin films irradiated by femtosecond pulses. However, thin‐film experiments are not reliable guides to a VO₂:Si system for all‐optical, on‐chip switching because of the differences in VO₂optical constants in the telecommunication band, and the complex sample geometry and alignment issues in a waveguide geometry. This paper reports the first demonstration that the reversible, ultrafast photoinduced phase transition in VO₂can achieve sub‐picosecond response when small VO₂volumes are integrated into a silicon waveguide as the active element. The result suggests that VO₂can be pursued as a strong candidate for waveguide switching with sub‐picosecond on‐off times.