We present images at 8.5, 10.5, and 12.5 microns which reveal that AFGL 2343 (HD 179821) is surrounded by a dusty nebula 4"-5" in diameter at 10.5 and 12.5 mum. This G5 Ia supergiant has long been suspected of having a detached dusty envelope because of its optical brightness and large IR excess. The nebula is ringlike and slightly elongated, with bright patches or clumps to the northeast. An 8.5 microns image is dominated by the photosphere of the central star, which also appears to a lesser extent in the 10.5 and 12.5 microns images. A full radiative transfer model of the nebula correctly predicts the observed inner (1"1) and outer (1"9) radii of the shell and the spectral energy distribution of the shell. With the aid of this model, we find that AFGL 2343 is almost certainly an extremely massive star at a distance of about 6 kpc, which has recently undergone an enormous mass-loss event, during which M⋅ = 10-2 Msun yr-1, while the current mass-loss rate is less than 2 × 10-6 Msun yr-1.