Strong electron localization into a bound state has been found in both paramagnetic and ferromagnetic states of the transition metal compound MnSi by muon spin-rotation spectroscopy in magnetic fields up to 7 T and from 2 K to room temperature. This bound state, with a characteristic radius R≈0.4 nm and net spin S=24±2, is consistent with confinement of the electron’s wave function within roughly one lattice cell of MnSi and is suggested to be a spin polaron. Such spin polarons may form due to a strong exchange interaction between itinerant electrons and the magnetic electrons of Mn ions of the same 3d type; as such, they might affect the peculiar electronic and magnetic properties of MnSi.