The German international ran the length of the field before firing the ball into the corner of the net to give Dortmund a 2-0 advantage after Pierre-Emerick Aubameyang had opened the scoring for the Bundesliga outfit.
Juventus were on the back foot early on, with Gianluigi Buffon called on to deny Kevin Kampl and Henrikh Mkhitaryan.
Juve newcomer Mario Mandzukic had a chance to give the Serie A champions the lead, but the Croatian headed Paulo Dybala's free-kick over the bar.
Having kept Aubameyang at bay on 27 minutes, Buffon was beaten by the striker's effort from the edge of the area after he had been picked out by a pinpoint pass from Mkhitaryan following a swift counterattack.
As is the custom in pre-season fixtures, both managers rung the changes at the break and the tempo of the match suffered as a result.
However, Reus sparked the game back into life on 64 minutes with a top draw effort. The midfielder picked up a loose ball on the edge of his area, dribbled the length of the field before carving out a shooting chance with some neat footwork and finding the corner with a fierce drive.
Dortmund had chances to make it three late on, but while they were unable to covert, they head into Thursday's Europa League qualifier against Wolfsberger on a high after recording their fifth win under new boss Thomas Tuchel