The Tinkoff-Saxo rider took the 158km stage from Mijas to Malaga in a stunning display of sprinting prowess, slingshotting himself past John Degenkolb after hitching a ride on the Giant-Alpecin train. Nacer Bouhanni (Cofidis) also overtook Degenkolb to take second on the stage.
Sagan has had little luck when competing for stage wins in Grand Tours over the last couple of years, despite Sagan's reputation as one of the strongest and most versatile sprinters in the peloton. his last stage win was Stage 7 of the 2013 Tour de France from Montpellier to Albi on the way to his second Tour de France green jersey.
He has been one of the most consistent finishers in the peloton since then, finishing in the top five in 27 Grand Tour stages, and second place in ten of those). While his performances have been enough to give him a stranglehold on the Tour's green jersey competition, crossing the line with his hands in the air in major stage races has eluded him until now.
There was no change in the Vuelta's general classification, with Orica-GREENEDGE's Esteban Chaves retaining the red jersey. Tom Dumoulin (Giant-Alpecin) and Nicolas Roche (Team Sky) hold second and third place respectively.
'You cannot always finish second'
Sagan was his usual idiosyncratic self when speaking to reporters after the stage, but he was clearly relieved to have finally crossed the finish line first.
'I’m very happy for this victory because I think it’s good for the entire team. This is a good start to the Vuelta a España. And finally I had a victory! I’m always trying to get the win and someday it has to come," he said.
'I’ve had many second places but you cannot always finish second. So today I finished first. That’s how cycling is, sometimes you’re up and sometimes you’re down.
Sagan: by the numbers
779 days since his last Grand Tour stage victory
72 Grand Tour stages raced since his last victory
27 top five finishes since his last victory
10 second places since his last victory
Sagan also pointed towards a potentially bigger objective than success in the Vuelta: victory in the World Championships in Richmond, USA in September. The rolling course is likely to suit a versatile rider with a fast finish, just like Sagan.
"I’m here at the Vuelta to do the best for my team and to get race kilometers into the legs before the World Championships. It’s good to know already that I have some speed in the legs and then we’ll see how it goes," added Sagan.
It might have been a day of joy for Sagan, but his Classics rival Fabian Cancellara's run of bad luck continued.
The Trek Factory Racing rider was forced to withdraw from the Vuelta with a stomach illness during Stage 3. Cancellara has already suffered two fractured vertebrae this year, ending his Classics and Tour de France campaigns. This withdrawal is likely to end his season, although there is an outside possibility he could still race the Worlds.
Astana rider Paolo Tiralongo also withdrew from the race as a result of injuries sustained in a crash on Stage 2 - an incident which ultimately saw Vincenzo Nibali ejected from the race for holding onto a team car.