We demonstrate a quantum nanophotonics platform based on germanium-vacancy (GeV) color centers in fiber-coupled diamond nanophotonic waveguides. We show that GeV optical transitions have a high quantum efficiency and are nearly lifetime-broadened in such nanophotonic structures. These properties yield an efficient interface between waveguide photons and a single GeV without the use of a cavity or slow-light waveguide. As a result, a single GeV center reduces waveguide transmission by 18 ± 1% on resonance in a single pass. We use a nanophotonic interferometer to perform homodyne detection of GeV resonance fluorescence. By probing the photon statistics of the output field, we demonstrate that the GeV-waveguide system is nonlinear at the single-photon level.Efficient coupling between single photons and coherent quantum emitters is a central element of quantum nonlinear optical systems and quantum networks [1][2][3]. Several atom-like defects in the solid-state are currently being explored as promising candidates for the realization of such systems [4], including the nitrogen-vacancy (NV) center in diamond, renowned for its long spin coherence at room temperature [5]; and the silicon-vacancy (SiV) center in diamond, which has recently been shown to have strong, coherent optical transitions in nanostructures [6][7][8][9]. The remarkable optical properties of the SiV center arise from its inversion symmetry [10], which results in a vanishing permanent electric dipole moment for SiV orbital states, dramatically reducing their response to charge fluctuations in the local environment.