The 23-year-old Frenchman Champoussin only turned professional a year ago, and with this first win, he raised a solitary finger in celebration before thumping his chest at the finish line.
Tour of the Basque Country: Tadej Pogacar fights off fellow Slovenian Primoz Roglic to win third stage
The ride to Ermualde finished with three sharp climbs in the final 20 kilometres. On the last one, Pogacar, who rides for Team UAE, took the lead and was quickly joined by Roglic of Jumbo-Visma.
Tour of Catalonia: Adam Yates wins tournament; Richie Porte, Geraint Thomas complete podium sweep for Ineos
This trio held on to the first three places despite efforts to dislodge them in the 133-kilometre closing stage around Barcelona, principally by home team Movistar's Alejandro Valverde.
UAE Tour: Tadej Pogacar manages lead to claim overall trophy, Australian Caleb Ewan wins final stage
The 22-year-old Pogacar dominated the race to turn the tables on Briton Adam Yates, who had edged the Slovenian in last year's coronavirus-curtailed edition of the World Tour event.
Bennett, riding for Deceuninck-Quick-Step, beat David Dekker, Caleb Ewan and Elia Viviani at the end of the 204-kilometre stage that started and ended at the tourist resort of Marjan Island.
Pre-Tour favourite Roglic has been the form man this season and after Sunday's second-place leads defending champion Ineos leader Egan Bernal of Colombia by 21 seconds.
Yellow jersey holder Adam Yates and other Tour contenders, including defending champion Egan Bernal and favorite Primoz Roglic, crossed the finish line 6 minutes, 40 seconds later.
Jumbo's Belgian all-rounder Van Aert also won stage five and on Friday outsprinted a small leading group.
Briton Adam Yates held on to the overall leader's yellow jersey despite a dramatic late bid by Frenchman Julian Alaphilippe to snatch it back with a trademark kick to the finish that clawed back a few seconds.
Tour de France 2020: Julian Alaphilippe relinquishes yellow jersey to Adam Yates; Wout Van Aert wins stage four
Mitchelton-Scott's Yates took advantage of Alaphilippe's 20-second sanction for drinking water from a bottle at the 17km mark as Belgium's Wout Van Aert won the fifth stage.
Road Cycling World Championships 2019: Great Britain's Geraint Thomas pulls out of men's team trial, to focus on team road race
Thomas remains in the six-man Great Britain team for Sunday's 285km road race from Leeds to Harrogate
Tour de France 2019: Britain's Simon Yates wins stage 12 as Geraint Thomas, Julian Alaphilippe finish together
Simon Yates is over an hour adrift in the overall standings, but punched the air in joy after outwitting his rivals to add this Tour de France stage to victories in the Vuelta and Giro