John Joseph "Jack" Hannahan IV (born March 4, 1980) is an American former professional baseball utility player.

Hannahan attended Cretin-Derham Hall High School in Saint Paul, Minnesota, and also played football as a defensive back and earned All-Conference honors in basketball.

[citation needed] The Detroit Tigers selected Hannahan in the third round of the 2001 Major League Baseball draft.

In 2008, Hannahan was on the major league roster the entire season, serving as the primary third baseman while Eric Chavez was on the disabled list most of the year.

[4][5] With the Mariners holding an 11–5 lead over the Kansas City Royals on August 6, Seattle manager Don Wakamatsu substituted Hannahan for shortstop Jack Wilson, who was experiencing minor discomfort in his right triceps area.

[7] Hannahan finished the season with a combined .213 batting average between the A's and the Mariners with 14 doubles, two triples, four home runs and 19 RBIs in 103 games.

On December 13, 2012, Hannahan signed a two-year, $4 million contract with a club option for 2015 with the Cincinnati Reds and was expected to provide depth throughout the infield, specifically at third base.

On December 23, 2014, Hannahan signed a one-year deal worth $1 million with the LG Twins of the KBO League.