William A. Adamaitis (January 25, 1915 – February 15, 1968) was an American football player who played college and professional football from 1933 to 1937.

Adamaitis played college football for the Catholic University Cardinals from 1933 to 1936. He led the Cardinals to a victory over the Ole Miss Rebels in the 1936 Orange Bowl and became the first, and one of only three players, to catch and throw a touchdown pass in the same Orange Bowl. He was selected to play on the College All-Star team in the Chicago College All-Star Game in August 1937 and led the college all-stars in their only scoring play against the NFL champion Philadelphia Eagles.

Although he had an offer to play for the Eagles, Adamaitis opted to remain in Washington, D.C., where he played professional football for the Washington Presidents of the Dixie League. Adamaitis helped lead the Presidents to the 1937 Dixie League championship with a 5–0–2 record in conference play.

Adamaitis was born in 1915 in Worcester County and attended Classicial High School in Worcester, Massachusetts, where he was captain of the basketball team, a member of the football, baseball and track teams. He received a bachelor's degree from The Catholic University of America in 1937 and a master's degree from Worcester State College in 1951.

Adamaitis played college football at the halfback position for The Catholic University of America Cardinals. During the 1933 season, he impressed observers with his passing ability, including a 30-yard completion, as he led Catholic University's frosh team to a victory over the University of Maryland's frosh team. He saw limited playing time in 1936, but became a star while playing for the varsity team during the 1935 and 1936 seasons.

During the 1935 season, Adamaitis was the leading scorer in the Washington District, which then included the Georgetown Hoyas, Maryland Terrapins, George Washington University and American University. In October 1935, Adamaitis's passing led the Cardinals to a 41–7 victory over a LaSalle team that had been undefeated since 1933. The Washington Post at the time noted that "Adamaitis played an important part in the slaughter by paving the way with bullet-like passes for several of the scoring jaunts."

The following week, Adamaitis was again led the Cardinals to a victory. The New York Times praised Adamaitis as the leader of the Cardinals' game-winning drive, "sprinting 36 yards to start the attack and passing to Peter Drangins for another fourteen-yard gain" before running the final three yards himself.

One week later, Adamaitis's passing led the Cardinals to a comeback victory over the University of Detroit. The Washington Post wrote that the "nonchalant halfback" completed nine passes for 158 yards, including the game-winners. The New York Times noted that, with the Cardinals trailing in the fourth quarter, Adamaitis threw a "long pass" that was caught by one player at the 12-yard, lateraled to another player at the eight-yard line, with the last player staggering across the goal line for the game-winning touchdown. The Post emphasized the accuracy of Adamaitis' passing: "He could have dotted an 'I' at 50 yards away today."