Jackie Delane Aker (born July 13, 1940) is an American former Major League Baseball right-handed relief pitcher. Aker was used exclusively in relief. He appeared in 495 games, none as a starter, and was one of the American League's best closers of his era. He is of Potawatomi Indian ancestry.

Jack Aker was originally signed as an outfielder by the Kansas City Athletics and led the Nebraska State League in stolen bases in his first year, 1959, before being converted to pitching before the 1960 season. He made it to the majors as a side-arming sinkerballer, pitching for the Kansas City/Oakland Athletics (1964–68), Seattle Pilots (1969), New York Yankees (1969–72), Chicago Cubs (1972–73), Atlanta Braves (1974), and New York Mets (1974). During an 11-year baseball career, Aker compiled 47 wins, 404 strikeouts, a 3.28 earned run average, and 123 saves, an impressive total at the time.

On September 10, 1965, Aker pitched 6.1 innings of relief, allowing just one run, to earn the W in a 10–5 win over the Baltimore Orioles

His best season came in 1966 with the A's, when he led the American League in saves (32, a major league record until 1970) and games finished (57), had a 1.99 ERA, finished 13th in the MVP voting, and was named AL Fireman of the Year by The Sporting News.

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