Out-jumping defenders at the near post, he glanced in an inswinging corner for his 14th goal of the season with just 16 minutes left to play.
In his delight he raced over to the corner and ripped out the flag, collecting a yellow card from top referee Steve Bennett.
The Memorial Stadium pitch, rated the worst in League One, had been heavily watered but the benefit of this took a long time to show.
Buoyed by the 6-0 thrashing of Port Vale at the weekend, Swindon should have taken control of the game inside the first half minute.
The Rovers defence opened up before Michael Timlin, allowing the on-loan Fulham midfielder a free shot, but from a central position in front of goal he pulled his shot well wide.
A similar chance quickly came Rovers' way when Swindon's left-back Miguel Comminges completed missed his clearance kick.
The ball was played in to Rickie Lambert who got power behind his shot, but was denied a goal by a great stop from Petr Brezovan.
In a nondescript first half, Swindon went on to owe a big debt to their keeper for two more saves from Lambert although his kicking was far from assured.
He was not alone, for Rovers' keeper Steve Phillips twice managed to put dead-ball efforts into touch.
Swindon had the better front combination in Lee Peacock and Cox and when Cox whipped in a cross from the left, Peacock's firm header was on target but well covered by the keeper.
After the scrappy opening period both teams were prepared to open up in the second half.
Byron Anthony, with a spirited burst, thought he had unlocked it for Rovers with two shots from a couple of yards. His first hit a defender, the follow-up a post.
The miss of the derby came later when Craig Disley lifted a shot over the bar after being put in the clear by a beautiful ball from Lambert.