Walter Henry Breen Jr. (September 5, 1928 – April 27, 1993) was an American numismatist, writer, and convicted child sex offender. 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. He was arrested in 1990 for child sex abuse and died in prison three years later.

From 1964 to 1990 Breen was married to popular science fiction and fantasy author Marion Zimmer Bradley; it was later revealed that Bradley had been aware of his crimes.

Breen was born in San Antonio, Texas, the son of Walter Henry Breen Sr. and Mary Helena (Nellie) Brown Mehl. He spent the first several years of his life in Texas with his parents.

At the time they met, both of Walter's parents were married to other people and living next door to each other in Parkersburg, West Virginia. Walter's father changed his own name from Walter H. Green to Breen after abandoning his wife and children to run away with Walter's mother. Later in life, Breen sometimes denied they were his birth parents and claimed to have been adopted by them as a foundling child. In reminiscences he spoke of being raised in a variety of "institutional and foster settings."

The 1940 census shows young Breen living in a Catholic orphanage in West Virginia, with his (by then) divorced mother living as a housekeeper in a Catholic church rectory less than two miles (3 km) away. Walter's father was by that time living with another woman in Chicago; for a while after their separation his mother resumed her maiden name and young Walter went by the name William Brown.

Breen strove to distinguish himself academically from a young age, attending a Catholic high school in Wheeling, West Virginia, and continued excelling academically throughout his postsecondary education. After being declared unfit for service by the United States Army Air Forces in April 1946, Breen was accepted that October with a recorded IQ of 144; following a severe beating, he was honorably discharged that December.

During his recovery, he read voluminously about rare coins and initiated correspondence with various members of the numismatics community, renewing his involvement in a hobby in which he had been actively engaged a few years earlier. Alternatively, Breen claimed that a severe head injury suffered in a World War II plane crash led to the development of his photographic memory.

He received his Bachelor of Arts in mathematics from Johns Hopkins University in 1952. He later claimed he finished four years of coursework in approximately ten months, concealing the fact that as a high-IQ teenage prodigy he had already completed two years at Georgetown University during World War II, followed by a brief stint at a small Catholic college in Texas. After graduating Phi Beta Kappa, he took a position as an auction cataloger for the New Netherland Coin Company while concurrently enrolled in pre-med courses at Columbia University, where he became a protege of the controversial psychologist and numismatist William Herbert Sheldon.