Reyna's goal came in the 35th minute of Dortmund's clash with Borussia Monchengladbach in the two sides' first match of the new season.
The goal was assisted by fellow teenager Jude Bellingham, who signed from Birmingham City this summer, as the English midfielder slid a pass into the path of the American star.
Reyna made no mistake with his far-post finish, giving Dortmund the 1-0 lead with his third senior goal.
GIO REYNA GOAL!!— ESPN (@espn) September 19, 2020
The 17-year-old American gets his first Bundesliga goal! pic.twitter.com/YpXoiIHX5c
Gio Reyna's first Bundesliga goal. Jude Bellingham's first Bundesliga assist.— Football on BT Sport (@btsportfootball) September 19, 2020
Two of Dortmund's teenage stars combine to score their first goal of the league season! 🌟 pic.twitter.com/jZjq80pZdG
At 17 years and 311 days old, Reyna becomes the second youngest American to score in the Bundesliga, following Christian Pulisic, who scored his first Dortmund goal at 17 years and 211 days old.
Saturday's big moment comes on the heels of a spectacular preseason which saw the young American lead the club with four goals.
Additionally, in the club's first official match of the season, a DFB-Pokal clash with Duisburg, Reyna scored the fourth of Dortmund's five goals, as he was joined on the scoresheet by Bellingham, Jadon Sancho, Thorgan Hazard and Marco Reus.
In that match, Bellingham became the youngest scorer in Borussia Dortmund's history at the age of 17 years and 77 days old.
Reyna started Saturday's Bundesliga opener as a No. 10, playing behind the likes of Sancho and Haaland, with the trio of young stars making up one of the world's most dynamic attacking units.
The American made his breakthrough last season after the Bundesliga's winter break, making 18 appearances across all competitions while scoring one goal in a DFB-Pokal loss to Werder Bremen.
Internationally, Reyna, the son of former USMNT star Claudio Reyna and former USWNT midfielder Danielle Egan, has represented the U.S. up to the Under-17 level and was due to be called up to the senior team this spring before the coronavirus pandemic shuttered international soccer.
Reyna isn't the first young American to seal a memorable start to the Bundesliga season as 20-year-old defender Chris Richards made an appearance as a substitute during Bayern Munich's 8-0 demolition of Schalke on Friday.