William Wakefield Cardinal Baum (born November 21, 1926 in Dallas, Texas) is the senior living Roman Catholic Cardinal from the United States and was the senior Cardinal Priest to participate in the 2005 Papal conclave.

Ordained a priest in 1951, he spent his early church career in Missouri. A privy chamberlain from 1961, from 1962 to 1965 he served as both vice-chancellor of the Diocese of Kansas City-St. Joseph and as an "expert" at the Second Vatican Council. He later became chancellor of that diocese before being named a bishop himself, heading the diocese of Springfield-Cape Girardeau from 1970.

Three years later Pope Paul VI elevated him to Archbishop of Washington, D.C. (his successor in Springfield-Cape Girardeau being the future Bernard Cardinal Law), and in the consistory of 1976 Baum was named a cardinal. He is one of three remaining cardinals elevated by Paul VI to retain voting rights in the coming election of a successor to Pope John Paul II.

Under John Paul, Baum's rise continued, as in 1980 he was named to the Roman Curia as Prefect of the Sacred Congregation for Catholic Education. In 1990 he became the Church's Major Penitentiary, exchanging the responsibility of overseeing church seminaries worldwide for that of supervising the office in charge of the forgiveness of sins.

Somewhat frail in recent years, he has suffered from deteriorating eyesight. His resignation as Penitentiary was accepted the day after his 75th birthday in 2001, but he has remained active in Rome to the extent that his health permits, and attended the meeting of American cardinals called to deal with the sex abuse scandal in 2003.

Only four American Cardinals in history (Gibbons, Dougherty, O'Connell, and Carberry) have had longer cardinalates than Baum.