John Joseph "Jack" Cust III (born January 16, 1979, in Flemington, New Jersey) is a designated hitter and outfielder for the Oakland Athletics of Major League Baseball. Cust attended Immaculata High School in Somerville, New Jersey. In 1997, he was first-team High School All-American at first base and did not attend college as he was drafted out of high school.

In 1997, Cust, coming out of high school, was the first round draft pick (30th overall) of the Arizona Diamondbacks, and had a .447 on-base percentage in 35 games in the Rookie League. Cust was a Pioneer League All-Star in 1998, where he led the league in walks (86), on-base percentage (.530), and runs scored (75). In 1999, he led the California League in homers (32), on-base percentage (.450), and slugging percentage (.651), and was Baseball America's 1st team Minor League All-Star DH, California League All-Star, and the Arizona Diamondbacks Minor League Player of the Year.

In 2000, he had a .440 on base percentage at El Paso in the Texas League while leading the league in walks (117) and strikeouts (150). In 2001, he was a Pacific Coast League All-Star while leading the league with 102 walks, and in 2002 he was the Triple-A All-Star Game MVP. In 2006, he led the PCL with 143 walks (also leading the minor leagues), walking 24.5% of the time, while sporting a .467 on-base percentage with 30 home runs.