Milton Arnold "Mickey" Haefner (October 9, 1912 – January 3, 1995) was an American knuckleball-throwing left-handed pitcher who played eight seasons in Major League Baseball between 1943 and 1950, six and a half of them with the Washington Senators (1943–1949), later joining the Chicago White Sox (1949–1950) and Boston Braves (1950).

He was born in Lenzburg, Illinois, and was listed as 5 feet 8 inches (1.73 m) tall and 160 pounds (73 kg), leading to his baseball moniker, Itsy-Bitsy.

In 1945, knuckleball aces Haefner, Dutch Leonard, Johnny Niggeling and Roger Wolff started 111 of the Senators' 154 games and accounted for 60 of the club's 87 victories, as Washington contended for the American League title up until the closing day of the season, before finishing 11⁄2 lengths behind the Detroit Tigers.

The Red Sox were idle while the Brooklyn Dodgers and St. Louis Cardinals engaged in the best-of-three 1946 National League tie-breaker series to determine the championship of the Senior Circuit.

Haefner earned a measure of notoriety when, during the tune-up exhibition game at Fenway Park on October 1, he accidentally plunked Red Sox superstar Ted Williams on the elbow with an errant knuckleball.