The optical detection of a large shell nebula, at least several hundred parsecs across, in NGC 55 is reported. The observation was made in the light of the forbidden O III 5007-A emission line using an image-tube camera together with a narrow-band interference filter. The observations confirm the gaseous nature of the nebula and its association with the galaxy. The observed nebulous arc is roughly centered on a bright stellar knot which appears to be an unusually active region of star formation. The shell is similar in form to the neutral-hydrogen supershells observed by Heiles (1979) in the Galaxy.