The 19-year-old, who has not featured in the Bundesliga since scoring in Dortmund's 3-2 win over Hoffenheim on Matchday 3 on 27 August, was able to take part in sections on the session on Tuesday, and will be gradually reintegrated over the coming days.
Reyna tore a muscle in his thigh while on international duty with the USMNT in a goalless draw with El Salvador on 3 September.
It was an untimely absence for the attacking midfielder, who began the season in sparkling form by providing two goals and an assist in his four competitive outings before being forced to the sidelines.
His return arrives at just the right time for BVB, who are currently without Jude Bellingham (knee), Raphael Guerreiro (muscular), Thorgan Hazard (illness) and Youssoufa Moukoko (eye), while top scorer Erling Haaland only recently made his comeback from a hip flexor injury.
Reyna is unlikely to feature in Dortmund's top-of-the-table Klassiker meeting at home to Bayern Munich this Saturday, but will be eased back into action over the coming weeks.