A new diagnostic is developed to reconstruct the plasma boundary using visible wavelength images. Exploiting the plasma’s edge-localized and toroidally symmetric emission profile, a new coordinate transform is presented to reconstruct the plasma boundary from a poloidal view image. The plasma boundary reconstruction is implemented in Matlab and applied to camera images of Mega-Amp Spherical Tokamak (MAST) discharges. The optically reconstructed plasma boundaries are compared to magnetic reconstructions from the offline reconstruction code EFIT, showing very good qualitative and quantitative agreement. Average errors are within 2cm and correlation is high. In the current software implementation, plasma boundary reconstruction from a single image takes 3ms. The applicability and system requirements of the new optical boundary reconstruction, called OFIT, for use in both feedback control of plasma position and shape and in offline reconstruction tools are discussed.