The 21-year-old's scorching 67th minute free kick proved decisive in a 2-1 victory which showed how far both he and Napoli have come in only two months of Rafael Benitez's reign at the club.
It was a phenomenal strike which flew in off the crossbar from a tricky angle.
"It was brilliant scoring my first goal in the Champions League here at the San Paolo against such a great team," he said.
"I'm happy that the goal won the game and that we'll be able to go to Milan (in Serie A on Sunday) with the right attitude.
"I can feel the faith the manager has in me."
Dortmund coach Juergen Klopp described it as "an exceptional goal".
In the past Napoli-born Insigne has often flattered to deceive, especially on the big occasion, but with the World Cup around the corner Wednesday's performance was a sign of a player looking to make the step up at the right time.
He could have had a goal after only 12 minutes when he cut inside and sent a curling shot just wide of the post.
His dribbling and distribution caused havoc on Dortmund's right flank, and he laid on a series of opportunities for his team mates.
Italy manager Cesare Prandelli lamented the lack of quality among Italy's young players during the recent international break, and many have noted the lack of creativity behind the strikers with no apparent heir to Alessandro Del Piero waiting in the wings.
Should Insigne continue to shine like he did on Wednesday he will almost certainly be given the chance to add to his two Italy caps.
- Sports & Recreation
- Rafael Benitez