William P. Melchionni (born October 19, 1944) is an American former National Basketball Association (NBA) and American Basketball Association (ABA) player. A three time All-Star, Melchionni is one of only four players to win NBA and ABA championships.

Melchionni was a guard from Bishop Eustace Prep (in his hometown of Pennsauken Township, New Jersey). His teams won two state championships.

Melchionni was a collegiate star in the mid-1960s at Villanova University and was the Most Valuable Player in the 1966 NIT, as Villanova finished in third place in the tournament.

Melchionni averaged 27.6 points as a senior at Villanova, as the team finished 18–11. As a junior, in 1964–1965, he averaged 19.4 points for the 23-5 Wildcats, who ended up #8 in the national rankings. In the 1966 season, the Wildcats won 9 of their last 10 games, finishing 3rd in the 1966 National Invitation Tournament, with Melchionni chosen as Tournament MVP. Melchionni scored 1,612 points during his career at Villanova.

Melchionni graduated in 1966 from Villanova with a B.S. Degree in economics.

Upon graduation from Villanova, Melchionni was a 2nd-round (19th) overall pick by the Philadelphia 76ers in the 1966 NBA draft. Melchionni joined the Phillips 66ers AAU Elite team before returning to his hometown to join the Philadelphia 76ers in 1966–1967.

Immediately, Melchionni was a key reserve on what is, statistically speaking, the greatest 76ers team ever, the 1967 NBA champions led by Wilt Chamberlain, Luke Jackson, Hal Greer, Chet Walker, Billy Cunningham, and fellow Villanova alum Wali Jones. The team was coached by another AAU Elite alum, Alex Hannum. Melchionni played in 71 games, averaging 4.3 points and 1.3 assists for the championship 76ers.

After another season in Philadelphia, Melchionni then played for the Trenton Colonials of the Eastern Professional Basketball League in 1968–1969.