The 28-year-old struck in the 32nd minute with a shot that went in off the underside of the bar after Andre Schuerrle and Mario Goetze combined to provide the opening.
Dortmund climbed to second place with 40 points from 23 games, though they will be overhauled if RB Leipzig, who are two points behind, win at Eintracht Frankfurt on Monday. Bayern Munich have a 19 point lead at the top.
Reus returned to action one week ago for the first time since suffering a knee injury in the German Cup final last May.
He is hoping that a good run of form can earn him a place in Germany's squad for this year's World Cup, having missed out on the tournament in Brazil in 2014 through another injury.
"I didn't hit it well. I really wanted to shoot it further to the left, towards the top corner, but the important thing is that the ball went in there," he said.
Gladbach were agonisingly close to an equaliser in the 55th minute when Lars Stindl's shot was parried by goalkeeper Roman Buerki and Raul Bobadilla's effort from the rebound was cleared off the line by Sokratis Papastathopoulos.
The Swiss keeper foiled Gladbach with further saves from Jonas Hofmann and a Nico Elvedi header.
VfB Stuttgart striker Mario Gomez, also hoping for a World Cup call-up by Germany, scored his second goal since returning to the club to secure a 1-0 win at Augsburg and pull them away from the relegation zone.
A free kick went straight into the Augsburg wall and rebounded to Gomez who scored with a typically opportunist effort in the 27th minute as Stuttgart won away from home for the first time this season.
Augsburg's Michael Gregoritsch thought he had levelled 10 minutes later but his goal was disallowed for offside by the video referee (VAR).
Stuttgart moved up to 13th, four points clear of the relegation playoff spot.
(Writing by Brian Homewood; Editing by Ken Ferris)
