Arthur Mahaffey, Jr. (born June 4, 1938 in Cincinnati, Ohio) is a former starting pitcher in Major League Baseball who played for the Philadelphia Phillies (1960-65) and St. Louis Cardinals (1966). He batted and threw right-handed. In a seven-season career, Mahaffey posted a 59-64 record with 639 strikeouts and a 4.17 ERA in 999.0 innings pitched.

Mahaffey was signed as an amateur free agent by the Philadelphia Phillies on June 29, 1956. After six seasons playing in the Phillies' minor league organization, Mahaffey was called up by the big league team. He played in his first game for the Phillies on July 30, 1960, pitching the final two innings of a game against the St. Louis Cardinals. Mahaffey retired the three batters he faced in the eighth inning. In the ninth, Bill White led off with a single to right field and then was picked off by Mahaffey at first base. The next batter, Curt Flood, singled to center field, and he too was picked off by Mahaffey, who threw to first with Flood tagged out on a throw from first to second. He finished the 1960 season with a 7-3 record and an earned run average of 2.31 in 14 games, finishing third in Rookie of the Year balloting that season, which was won by Frank Howard of the Los Angeles Dodgers.

He set a club record with 17 strikeouts in a game against the Chicago Cubs on April 23, 1961. Though he ended the 1961 season with an ERA of 4.10 and a record of 11-19 in 36 games, leading the National League in losses, he was selected to represent the Phillies on the National League All-Star team. He ended the 1962 season with a record of 19-14, and ERA of 3.94 and a career high 177 strikeouts in 41 games. He was selected again in 1962 for the All-Star team and finished 26th in balloting for Most Valuable Player, despite leading the league in home runs allowed with 36 and earned runs allowed with 120. Mahaffey had a 7-10 record in 26 games with the Phillies in 1963, to go along with a 3.99 ERA. In 1964, he finished the season with a record of 12-9 and an ERA of 4.52 in 34 games. The 1964 team had a 61⁄₂-game lead in first place with 12 games remaining in the season, before starting a 10-game losing streak that cost the team the pennant. Mahaffey pitched in two of the games as part of the losing streak, losing a 1-0 game (the first of that losing streak) on a steal of home by Chico Ruiz of the Cincinnati Reds, and was taken out while winning in a game against the Braves in which Rico Carty hit a triple off of reliever Bobby Shantz to win the game. 1965 was his last season in Philadelphia, which saw him finish with a 2-5 record and an ERA of 6.21 in 22 games, mostly in relief.

He was traded by the Phillies on October 27, 1965, along with catcher Pat Corrales and outfielder Alex Johnson to the St. Louis Cardinals, in exchange for shortstop Dick Groat, catcher Bob Uecker and first baseman Bill White. In his only season with the Cardinals, he had a 1-4 record in 12 games with an ERA of 6.43. His final major league game was on July 17, 1966, in the second game of a doubleheader against the Chicago Cubs, in which Mahaffey started and gave up three hits and three runs in 1⁄₃ of an inning, in a game the Cardinals lost by a score of 7-2.

The Cardinals traded him on April 1, 1967 along with infielder Jerry Buchek and shortstop Tony Martínez to the New York Mets in exchange for shortstop Eddie Bressoud, Danny Napoleon and cash, though he never played for the Mets.

Mahaffey now resides in Allentown, Pennsylvania.