We present Hα images of an ionized nebula surrounding theM2-5Ia red supergiant (RSG)W26 in the massive star cluster Westerlund 1. The nebula consists of a circumstellar shell or ring ̃0.1 pc in diameter and a triangular nebulã0.2 pc 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 suggests that W26 is a highly evolved RSG experiencing extreme levels of mass-loss. As the only known example of an ionized nebula surrounding an RSGW26 deserves further attention to improve our understanding of the final evolutionary stages of massive stars.©2013 The Authors.