A firearm magazine adapter to be affixed to a non-native magazine to allow the adapted magazine to fit and function in a firearm that normally cannot accept the non-native magazine. Embodiments comprise two or three primary elements: two clamshell halves for the body or a one-piece body, and an ejector which is captive, for use with a non-native magazine, in the body. In one embodiment the magazine adapter is free of moving parts. In one embodiment, two screws hold the clamshell halves together and secure the ejector via a partial cylindrical negative volume. In use, the assembly is permanently and removably affixed to a non-native magazine. A single, unmodified release on the firearm releases the adapted magazine from the firearm.