The Lotto-Belisol rider held off Garmin-Sharp's Tyler Farrar and Team Sky's Ben Swift to take the honours in the race, which consisted of 10 laps of a 11.6km circuit in the centre of the Balearic island's capital.
It was a first win in five years for Dehaes, who overhauled Farrar and Swift in the final 200m.
Perrig Quemeneur of Europcar and Fran Moreno of Caja Rural tried an early breakaway and pulled out a four-minute lead in the race, traditionally dominated by sprinters, but they were caught with 20km to go.
Dehaes said: “We knew this stage usually ends with a bunch sprint, so before the race we agreed that we would give me a chance, which turned out perfectly. After a turn of 180 degrees we had a headwind for the last two kilometres, so we had to wait pretty long.
"Dennis Vanendert got us perfectly to that turn, but then I lost Vicente Reynes. At 800 meters before the finish I was rather far, but Gaëtan Bille did a brilliant effort to bring me back in front, where I could place myself in the wheel of Farrar.
"I felt someone was coming from the left, so I decided to start sprinting, because In the past I sometimes let others close me in. And luckily I could hold on; the puzzle pieces fell perfectly into place.”