Borussia Dortmund 2019–20 season: Breakthrough On January 18, 2020, Reyna made his
Bundesliga debut for
Borussia Dortmund, coming on as a substitute in the 72nd minute, in a 5–3 win against
FC Augsburg. Hence, he became the youngest American, aged 17 years and 66 days, ever to appear in the Bundesliga, breaking a record previously set by
Christian Pulisic. On February 18, 2020, Reyna became the third-youngest player ever to appear in a knockout game of the
UEFA Champions League when he came on as a substitute in the 67th minute against
Paris Saint-Germain. Nine minutes later, Reyna set up
Erling Haaland's game-winning goal for Borussia Dortmund in the first leg of the Round of 16, becoming the youngest American to play and record an assist in a Champions League fixture. On May 16, Reyna was set to start his first Bundesliga match for Borussia Dortmund in their derby against
Schalke 04, but suffered a calf injury during warm-ups. Reyna returned on May 23 and played in the final 11 minutes of a 2–0 victory at
VfL Wolfsburg.
2020–21 season: DFB-Pokal win On September 14, 2020, Reyna started in a
DFB-Pokal match and scored with a free kick in a 5–0 win against
MSV Duisburg. On September 19, 2020, Reyna scored his first Bundesliga goal in a 3–0 win against
Borussia Mönchengladbach, aged 17 years and 311 days, to become the second youngest American scorer in Bundesliga history behind
Christian Pulisic, who was aged 17 years and 211 days.On October 3, 2020, in his third league appearance of the 2020–21 season, Reyna assisted 3 goals—a brace to
Erling Haaland and an
Emre Can header from a corner kick, to beat
SC Freiburg 4–0. As a result, Reyna became the first American to record a
hat trick of assists in one of the 5 major European leagues since
Steve Cherundolo did it for
Hannover in March 2008, and the youngest player to ever do so in the Bundesliga. On December 5, Reyna scored his second
Bundesliga goal against
Eintracht Frankfurt, with a near post finish just inside the 18-yard box. This goal made him the second youngest American to score twice in the league after
Christian Pulisic did so for the same club as Reyna,
Borussia Dortmund. On December 22, he was voted the U.S. Soccer's Young Male Player of the Year for 2020. Reyna was a second-half substitute for Dortmund in their 4–1 victory over
RB Leipzig in the
2021 DFB-Pokal Final on May 13, 2021. His involvement in the match marks him as the youngest American to appear in a domestic cup final in Europe, a record previously held by his international teammate Christian Pulisic. For his strong youth performances, he was on the final shortlist for the 2021
Golden Boy; finishing 6th, and
2021 Kopa Trophy; finishing 9th. On August 27, he became the youngest player to make 50
Bundesliga appearances during the league match against
Hoffenheim. Reyna suffered a series of injuries that caused him to miss the majority of the
2021–22 season. On September 6, 2022, Reyna became the first American player to provide two assists in a Champions League match in a 3–0 win against
Copenhagen. Returning from injuries, on October 22, he scored his first Bundesliga goal in 421 days, adding to a 5–0 victory against
VfB Stuttgart. Returning from the
2022 FIFA World Cup, on January 22, 2023, he scored a game-winning goal off the bench against
FC Augsburg in the 78th minute, winning 4–3. Three days later, he scored another game-winning goal off the bench in stoppage time in the 93rd minute against
Mainz 05, winning 2–1. Coming into his final season match against Mainz, Reyna had the second best minute-per-goal ratio of a
top five European league player for the 2022–23 season after
Erling Haaland. On May 27, during the final game of the
2022–23 Bundesliga where Dortmund was in first place, Reyna came off the bench in the 63rd minute when Dortmund were 2–0 down against
Mainz 05 and created two assists to level the game 2–2. The game ended in a draw and Dortmund was the league runner-up to Bayern. Reyna finished the season with four assists and his highest Bundesliga goal amount with seven goals, five of which were as a substitute.
2024–2025: Loan and playtime decline On January 31, 2024, Reyna extended his contract with Borussia Dortmund until June 30, 2026, and was subsequently loaned to
Premier League club
Nottingham Forest until the end of the
2023–24 season. He made his debut for Forest as a 78th-minute substitute for
Anthony Elanga in a 1–1 draw against
Bournemouth in the
Premier League on February 4. On April 13, Reyna recorded an assist against
Wolverhampton Wanderers in his first start of the season. He ultimately only featured in 10 matches for Forest and returned to Dortmund, finishing the 2023–24 season with no goals for both clubs. Reyna played only sporadically throughout the
2024–25 season, playing only 341 league minutes and scoring two goals with one assist.
Borussia Mönchengladbach On August 23, 2025, Reyna made a permanent move to
Borussia Mönchengladbach until 2028 for around €7 million. ==International career==