On November 17, 2014, the Braves traded Heyward to the St. Louis Cardinals along with pitcher Jordan Walden for pitchers Shelby Miller and Tyrell Jenkins to replace their former right fielder and top prospect Oscar Taveras, who died in a car accident a month earlier.
Cardinals manager Mike Matheny, who wore uniform #22 and had also done so for most of his playing career, gave his number to Heyward.
Heyward wears this number to honor his friend Andrew Wilmot.
After the trade was announced, Heyward published a Twitter message thanking the Atlanta Braves organization and fans for their support.

Five years to the day of his MLB debut, Heyward appeared in his first game as a Cardinal against the Chicago Cubs on April 5, 2015.
He garnered three hits, including two doubles and a stolen base in a 3–0 victory.
His first home run as a Cardinals player was on April 18 against the Cincinnati Reds at Busch Stadium with a 5–2 win.
After uncharacteristic errors in both of the two previous games, his ninth-inning home run against the Arizona Diamondbacks on May 27 tied the game 3–3, and the Cardinals eventually won with a score of 4–3.
 He homered in three straight games June 22–24, including successive contests against the Marlins from June 23–24 as St. Louis won both times.
On July 18 against the Mets, he matched a career high with five hits in a 12–2 win.

In an August 16 contest against the Marlins, Heyward hit two home runs for his first multi-home run game with the Cardinals.
An important defensive play was on September 20 against the Cubs.
With the bases loaded and no outs in the eighth inning, Addison Russell hit a fly ball that Heyward caught running and threw home to catcher Yadier Molina to tag out Anthony Rizzo by two steps, preserving a 4–3 win.
In the second game of a doubleheader against Pittsburgh on September 30, Heyward hit his second career grand slam in an 11–1 win, giving the Cardinals their 100th victory of the season while clinching their third consecutive National League Central division title.
He also robbed both Francisco Cervelli and Michael Morse of hits in this game.

Heyward finished the season with a career-high .293 batting average, .359 on-base percentage, and .439 slugging percentage.
Since his major league debut, he also had led all major league fielders in DRS and, for the previous three seasons, was second only to former Braves teammate Andrelton Simmons.
Among all outfielders since 2010, Heyward's 96.2 accumulated ultimate zone rating (UZR) led all major league outfielders to second-place Alex Gordon's 68.3 UZR.
After becoming a free agent for the first time in his career, Heyward won his third Fielding Bible Award and third Gold Glove Award.
On November 17, 2014, the Braves traded Heyward to the St. Louis Cardinals along with pitcher Jordan Walden for pitchers Shelby Miller and Tyrell Jenkins to replace their former right fielder and top prospect Oscar Taveras, who died in a car accident a month earlier. Cardinals manager Mike Matheny, who wore uniform #22 and had also done so for most of his playing career, gave his number to Heyward. Heyward wears this number to honor his friend Andrew Wilmot. After the trade was announced, Heyward published a Twitter message thanking the Atlanta Braves organization and fans for their support.
Five years to the day of his MLB debut, Heyward appeared in his first game as a Cardinal against the Chicago Cubs on April 5, 2015. He garnered three hits, including two doubles and a stolen base in a 3–0 victory. His first home run as a Cardinals player was on April 18 against the Cincinnati Reds at Busch Stadium with a 5–2 win. After uncharacteristic errors in both of the two previous games, his ninth-inning home run against the Arizona Diamondbacks on May 27 tied the game 3–3, and the Cardinals eventually won with a score of 4–3.  He homered in three straight games June 22–24, including successive contests against the Marlins from June 23–24 as St. Louis won both times. On July 18 against the Mets, he matched a career high with five hits in a 12–2 win.
In an August 16 contest against the Marlins, Heyward hit two home runs for his first multi-home run game with the Cardinals. An important defensive play was on September 20 against the Cubs. With the bases loaded and no outs in the eighth inning, Addison Russell hit a fly ball that Heyward caught running and threw home to catcher Yadier Molina to tag out Anthony Rizzo by two steps, preserving a 4–3 win. In the second game of a doubleheader against Pittsburgh on September 30, Heyward hit his second career grand slam in an 11–1 win, giving the Cardinals their 100th victory of the season while clinching their third consecutive National League Central division title. He also robbed both Francisco Cervelli and Michael Morse of hits in this game.
Heyward finished the season with a career-high .293 batting average, .359 on-base percentage, and .439 slugging percentage. Since his major league debut, he also had led all major league fielders in DRS and, for the previous three seasons, was second only to former Braves teammate Andrelton Simmons. Among all outfielders since 2010, Heyward's 96.2 accumulated ultimate zone rating (UZR) led all major league outfielders to second-place Alex Gordon's 68.3 UZR. After becoming a free agent for the first time in his career, Heyward won his third Fielding Bible Award and third Gold Glove Award.
Jason Heyward