Square porphyrin nanosheets with uniform thickness (7-12 nm) and aspect ratios up to 100 have been synthesized for the first time using a reprecipitation method. They retain the photocatalytic activity of the precursor molecules and can be self-metallized with Pt, Pd, and Au. Their unique morphology, photocatalytic properties, and large surface areas suggest potential applications in photo- and electrocatalysis, sensing, as components for optoelectronic nanodevices, and solar energy capture and conversion.