Walter Henry Breen Jr. (September 5, 1928 – April 27, 1993) was an American numismatist, writer, and convicted child sex offender; as well as the husband of author Marion Zimmer Bradley. He was known among coin collectors for writing Walter Breen's Complete Encyclopedia of U.S. and Colonial Coins. "Breen numbers", from his encyclopedia, are widely used to attribute varieties of coins. He was also known for activity in the science fiction fan community and for his writings in defense of pederasty as a NAMBLA activist.