A narrow-band Hα image and high-resolution spectroscopy have been obtained in order to investigate the nature of S266 and its central star MWC 137. The analysis of the stellar and nebular spectra suggests that MWC 137 is a B[e] supergiant located >= 6 kpc away from the Sun and not a Herbig Ae/Be star, as it has been traditionally classified. Moreover, the morphology and other properties of the nebula suggest that S266 is a ring nebula, probably produced by the interaction of stellar winds with the ambient interstellar medium or unprocessed ejected matter. This result would imply that S266 is the first ring nebula around a B[e] supergiant known in the Galaxy.