  Integrating solid-state quantum emitters with photonic circuits is essential
for realizing large-scale quantum photonic processors. Negatively charged
tin-vacancy (SnV$^-$) centers in diamond have emerged as promising candidates
for quantum emitters because of their excellent optical and spin properties
including narrow-linewidth emission and long spin coherence times. SnV$^-$
centers need to be incorporated in optical waveguides for efficient on-chip
routing of the photons they generate. However, such integration has yet to be
realized. In this Letter, we demonstrate the coupling of SnV$^-$ centers to a
nanophotonic waveguide. We realize this device by leveraging our recently
developed shallow ion implantation and growth method for generation of
high-quality SnV$^-$ centers and the advanced quasi-isotropic diamond
fabrication technique. We confirm the compatibility and robustness of these
techniques through successful coupling of narrow-linewidth SnV$^-$ centers (as
narrow as $36\pm2$ MHz) to the diamond waveguide. Furthermore, we investigate
the stability of waveguide-coupled SnV$^-$ centers under resonant excitation.
Our results are an important step toward SnV$^-$-based on-chip spin-photon
interfaces, single-photon nonlinearity, and photon-mediated spin interactions.
