William John Castagna (born June 25, 1924) is an American lawyer and judge.
Castagna was born in Philadelphia. He served in the United States Air Force from 1943 to 1945. He received his LL.B. from the University of Florida in 1949. He was in private practice in Miami, Florida from 1949 to 1950 and in Clearwater from 1950 to 1979. He received his J.D. from the University of Florida College of Law in 1967. United States President Jimmy Carter appointed Castagna to the United States District Court for the Middle District of Florida on June 5, 1979, to a new seat created by 92 Stat. 1629. Confirmed by the Senate on July 23, 1979, and received commission a day later. Castagna assumed senior status on June 29, 1992.