Jack Flaherty (born October 15, 1995) is an American professional baseball pitcher for the St. Louis Cardinals of Major League Baseball (MLB). He was drafted by the Cardinals in the first round of the 2014 Major League Baseball Draft.
Flaherty attended Harvard-Westlake School in Los Angeles, California. He pitched and played shortstop and third base as a member of the varsity baseball team for all four of his years at Harvard-Westlake. He was a sophomore when two of his high school teammates and fellow pitchers Max Fried and Lucas Giolito were both drafted in the first round of the 2012 MLB Draft. During his senior season, Harvard-Westlake was the consensus #1 team in the nation in the MaxPreps "XCellent 50" for much of the beginning of the season, before dropping out of the rankings altogether. 
On April 29, 2014, Flaherty pitched a complete game, 2–1 victory over Bishop Alemany High School to bring a two-year win–loss record to 20–0 (W–L).  In his last two seasons, his record was 23–0 overall.  He finished the 2014 regular season with an 0.69 ERA and 13–0 record on his way to being named Southern Section Division I player of the year. He pitched 78 innings (IP), struck out 125 (SO) batters while giving up just 32 hits and 12 bases on balls (BB). In the first game of the playoffs – which was also the last game of his high school career – he threw a no-hitter against Riverside North. He was subsequently named the 2014 Gatorade Player of the Year in baseball for the state of California.   The Los Angeles Times selected him as their baseball player of the year. In his four years on the mound, his record was 35–3. 
Although Flaherty signed a letter of intent to attend the University of North Carolina,  he began his professional career after the Cardinals drafted him in the first round with the 34th overall selection of the 2014 MLB Draft. It was a compensatory pick that the Cardinals gained when Carlos Beltrán signed with the New York Yankees as a free agent.  Flaherty signed with the Cardinals on June 17 for a $2 million bonus  and began his minor league career with the Cardinals of the Gulf Coast League, where he posted a 1.59 ERA in 22.2 innings pitched. In the July 2014 update of MLB.com's Prospect Watch, he was rated as the fifth-best prospect in the Cardinals farm system.  Prior to the 2016 season, Baseball America ranked him as the third-best prospect in the Cardinals' system.  Flaherty spent 2015 with the Peoria Chiefs where he posted a 9-3 record with a 2.84 ERA.
In their updated 2016 mid-season ranking, Baseball America rated Flaherty in the top-100 for the first time, at 88th. 
Flaherty spent the 2016 season with the Palm Beach Cardinals where he posted a 5-9 record with a 3.56 ERA.
He then began the 2017 season with the Springfield Cardinals, and after posting a 7-2 record with a 1.42 ERA, he was promoted to the Memphis Redbirds posting a 2.74 ERA in 15 starts. Combined, Flaherty has struck out 147, and walked 35 in 148 2⁄ 3 innings 
On September 1, 2017, Flaherty was promoted to the majors to make his MLB debut against the San Francisco Giants.  He pitched four innings, gave up five runs, and struck out six while not receiving a decision in an eventual 11−6 win.  He struck out the first batter he faced, Denard Span.