Prohibition of Undetectable Firearms Act - Amends the Federal criminal code to prohibit the manufacture or importation of plastic firearms unless the polymer portions of the frame or receiver, barrel, and slide or cylinder have been infused with a compound sufficient to render such firearm capable of being detected by airport security x-ray systems, and: (1) the barrel of such firearm is constructed of a metallic substance susceptible to being detected by a magnetometer; or (2) certain identifying information is engraved, cast, or stamped in a metallic substance susceptible to being detected by a magnetometer.

Defines the term "plastic firearm" to mean a firearm, the frame or receiver and slide (or the frame or receiver and cylinder) of which is constructed entirely of polymer or ceramic.

States that no provision of this Act shall apply to any firearm manufactured, imported, or possessed prior to the date of enactment of this Act.