The young star scored two brilliant efforts on the way to a drubbing of the visitors, moving BVB level with Schalke in the 2012-13 Bundesliga after six games
The forward was tormentor-in-chief against his former club, scoring twice either side of Nevan Subotic's header before the rout was completed by Ilkay Gundogan and Jakub Blaszczykowski to ensure the Bundesliga holders would enjoy themselves on Saturday afternoon.
Needing a win to keep touch with the Bundesliga pace-setters after a mixed start to the season, the champions were almost shocked barely three minutes into the game when Mike Hanke had the goal in his sights. The Monchengladbach striker was played clean through by Mario Gotze's unfortunate deflection, but Roman Weidenfeller was alert and came out well to end the danger for the hosts.
Both teams pushed forward during an even, keenly-contested first half. Juan Arango came within inches of connecting with a deep corner within the opening exchanges, while at the other end Nevan Subotic also experienced set-piece anguish. The centre-back released an excellent header that looked destined for goal, but the crossbar denied him the chance to open the scoring to the relief of a Gladbach side whose marking had been questionable from the corner.
As the half progressed, it was Dortmund who looked the most likely to bother the score-keepers, and with 10 minutes remaining before the interval they took the lead through the excellent Reus. Picking up the ball well outside the rival area, the forward skipped past the defence before calmly finishing past goalkeeper Marc Andre ter Stegen, handing BVB the advantage.
The goal was a hammer blow for Gladbach so close to the break, and there was worse to come. Once again Subotic rose higher than anyone to connect with a corner from the right-hand side, and this time the defender made no mistake as he guided the ball past the despairing dive of Stegen to make it 2-0.
Having matched Dortmund for almost all of the first period, the away team were punished for their inattention in defence and left with a mountain to climb if they wanted to take something from the match.
Having surrendered an identical advantage against Eintracht Frankfurt, Dortmund came close to seeing their lead pegged back just after the interval. Weidenfeller was again the hero, acrobatically denying Igo de Camargo from close-range after the ball broke to the striker in the area.
Both sides pushed forward freely in what was an open start to proceedings, Gotze in particular looking to make the difference for his side with a handful of dangerous forays into away territory.
With 20 minutes left on the clock, the game was decided in spectacular fashion. Reus had appeared to squander a decent chance for Dortmund when his attempted one-two failed to pay off, but the 23-year-old simply picked up the loose ball and unleashed a looping effort that gave Stegen no chance. 3-0 up, the home team could taste a valuable victory.
The final minutes were little more than a procession for the home team, who were in complete control and continued pushing for more against a beleagured Monchengladbach. The visitors' woes were compounded 10 minutes from the end, when Stegen was taken off injured and replacement Christofer Heimeroth had to pick the ball out of the end following Ilkay Gundogan's routine close-range finish.
And the hosts were not finished; Jakub Blaszczykowski notched the fifth in the final stages to complete Gladbach's humiliation and seal a win that takes Dortmund to 11 points from their opening six domestic fixtures, moving them level with Schalke in third. Gladbach, meanwhile, remain mired in the bottom half of the table with just six points from as many games in 2012-13.