The French Total Energies rider sped down the left into the final corner with 300 metres to go before out-sprinting Orluis Aular and Edward Theuns, with Australian Kaden Groves finishing in fifth place as he tried to win his third stage of the tour.
"I didn't think it was possible to win a stage because it's really, really, really fast in the sprint," Soupe said.
"Yet today it's really nervous in the final (sprint), we have a lot of roundabouts, a lot of wind also. It was a long shot, a surprise, but always special."
A crash in the middle of the bunch with 10km to go brought a few riders down, including American Sepp Kuss, but the winner of stage six was immediately back in the race.
Five km later another high-speed crash at the front of the race saw Dutchman Thymen Arensman (Ineos Grenadiers) taken down.
The seventh stage was a largely flat 201 km ride from Utiel to Oliva and, after two riders attacked and broke away in the opening kilometre, the final 40 km saw the bunch ease their way to the finish as the teams lined up their sprinters.
Ander Okamika (Burgos-BH) and Jose Herrada (Cofidis) opened a gap of two minutes 35 seconds at one stage but a puncture for Okamika allowed the peloton to close. Herrada was caught with 67km to go and Okamika was finally reeled in with 41km left.
Groves, winner of stages four and five, managed to win the intermediate sprint to maintain his grip on the Green jersey.
Saturday's stage eight is a mountainous 165 km ride from Denia to Xorret de Cati.