  We present H\alpha images of an ionized nebula surrounding the M2-5Ia red
supergiant (RSG) W26 in the massive star cluster Westerlund 1. The nebula
consists of a circumstellar shell or ring ~0.1pc in diameter and a triangular
nebula ~0.2pc from the star that in high-resolution Hubble Space Telescope
images shows a complex filamentary structure. The excitation mechanism of both
regions is unclear since RSGs are too cool to produce ionizing photons and we
consider various possibilities. The presence of the nebula, high stellar
luminosity and spectral variability suggest that W26 is a highly evolved RSG
experiencing extreme levels of mass-loss. As the only known example of an
ionized nebula surrounding a RSG W26 deserves further attention to improve our
understanding of the final evolutionary stages of massive stars.
