<p>Nonlinear waveguides harnessing the optical Kerr effect are promising for next-generation photonic technologies due to its ultrafast response, but the weak nonlinearities reported so far have limited practical applications. Here, we explore free-electron-induced Kerr nonlinearities in all-semiconductor waveguides, and we show that longitudinal bulk plasmons—nonlocal excitations—can induce exceptionally strong Kerr nonlinearities. Combining a novel nonlinear eigenmode analysis with semiclassical hydrodynamic theory, we compute the linear and nonlinear optical responses originating from the quantum behavior of free electrons in heavily doped semiconductors. Our waveguides achieve ultrahigh nonlinear refractive indices <i>n</i>₂ = −8.93 × 10⁻¹⁶ m² W⁻¹ and nonlinear coefficients <i>γ</i>_wg = 4 × 10⁷ W⁻¹ km⁻¹ while supporting modes propagating over 100 μm, all robust under viscoelastic and nonlinear dampings. Finally, we demonstrate efficient nonlinear modulation of the transmittance spectrum of a Mach–Zehnder interferometer made of our all-semiconductor waveguides via the free-electron Kerr effect. This work evidences the transformative potential of free-electron nonlinearities in heavily doped semiconductors for integrated photonics and scalable on-chip nonlinear systems.</p>