Junior, who turned down a contract to remain at Walsall during the summer, returned to haunt the Saddlers with his first goal for Derby County.
The Brazilian scored the vital winner in the 80th minute to give the Rams their first away win of the season.
The golden-haired striker was often jeered by the home fans but he had the last laugh with his vital strike.
Derby had often been the more dangerous side and only the brilliance of goalkeeper James Walker kept the Rams at bay.
But in the 80th minute a cross from Paul Boertien was collected by Ian Taylor. The Derby skipper's shot was blocked but fell into the path of Junior.
The Brazilian turned and scored with a low left-foot shot to the dismay of the Saddlers fans.
Junior quickly posted his intentions and had an early opportunity to shine against his former club when Lee Morris broke down the left to provide a cross which the Brazilian headed against the post.
A couple of minutes later Junior lashed in a free kick which Walker was forced to flick over the bar.
In this spell bottom of the table Derby were often the more dangerous side and only some superb defensive work by Paul Ritchie kept Walsall in the game.
The Saddlers were then indebted to the quick thinking of Walker as he dashed from his line to prevent Morris getting in a shot.
There were some good moves inspired by Paul Merson but the former England international could not carve out a goal against a resolute Derby defence in which goalkeeper Andy Oakes did not have to make a single first-half save.
Two minutes into the second half, however, Oakes distinguished himself with a brilliant save to keep out a Jorge Leitao header following an accurate cross from Merson.
Derby's 16-year-old well-built midfielder, Tom Huddlestone, impressed with another performance which belied his youth.
But Junior became Derby's hero with his late winner to the dismay of the Walsall fans.
Derby manager George Burley was delighted with the result saying: "We have been playing better of late and this win has been coming for some time.
"Junior took his goal well and I am pleased with the way he played." Walsall's manager, Colin Lee, was far from pleased saying: "It was a poor game and our worst performance so far this season."