Campenaerts was in the leading trio with Michal Kwiatkowski and Matteo Vercher, with the Belgian biding his time behind the other two riders before exploding into a powerful sprint in the final 200 metres to take his first-ever Tour stage win.
Campenaerts was emotional after the victory as he immediately got on a video call with his girlfriend and child.
"I've been dreaming about this for a very, very long time," he said, tearing up and struggling to speak.
"After the Classics, I had a very difficult time. I had a verbal agreement with the team about extending the contract and I got ignored for a long time and it was really difficult.
"I was on a long altitude camp but my girlfriend was there and she supported me every day, highly pregnant, and I was struggling to finish my training schedules.
"But I changed my mind, I have a bright future now still in cycling. I became a father and I saw only blue skies."
Trio breakaway
Campenaerts, Kwiatkowski and Vercher broke away and surged ahead about 35 km from the finish and the trio kept the pressure on as a group of five riders attempted to catch up.
But they managed to hold the chasing pack at bay with a comfortable pace up front until the final sprint where Campenaerts hung back while Kwiatkowski led, constantly looking over his shoulder.
Campenaerts eventually kicked into gear to overtake both riders who gave chase in vain as TotalEnergies' Vercher, who crashed earlier in the neutral zone at the start of the stage, finished second and Kwiatkowski of Ineos Grenadiers third.
"She (girlfriend) is the hero in this story. I'm so grateful that she made this possible, that I had a super long altitude camp to prepare for this Tour de France," Campenaerts added.
"I had faith that I would be in good shape. I will be leaving the team, but I'm so happy that I can finish off with maybe the highlight of my career. We will celebrate tonight!"
The chasing pack with riders such as Jai Hindley and Wout Van Aert eventually crossed the line 22 seconds behind.
Tadej Pogacar and the yellow jersey group finished nearly 14 minutes behind as they played it safe and conserved their energy ahead of the next two mountain stages.
Slovenian Pogacar retained the yellow jersey and leads Jonas Vingegaard by three minutes and 11 seconds.
The Tour now heads into the Alps and Friday's stage 19 is a 145 km ride from Embrun to Isola 2000.
