Muon spin-rotation experiments (supported by magnetization measurements) have been carried out in the canonical 4f mixed-valence narrow-gap semiconductor SmS from 10 to 900 K in magnetic fields up to 3.5 T. A bound state of an electron around a positive muon is found to form up to about 800 K. This state is a magnetic polaron: the electron wave function is confined within R≈0.5 nm (the first two coordination spheres) due to its exchange interaction with Sm magnetic moments. As such, it may serve as a model system for the hypothetical bound state suggested to account for a transition from divalent Sm²⁺ to trivalent Sm³⁺, which is invoked to explain the transformation of SmS from a paramagnetic insulator into a magnetic metal at high pressure.