The win marked the first major victory for the Jayco AlUla rider in a Grand Tour.
When Dunbar accelerated ahead of the lead group, no one could keep up. Despite the Irishman’s small build, he successfully fended off the more powerful riders from the day's breakaway.
Dunbar triumphed over Belgian Quinten Hermans of Alpecin-Deceuninck, who secured second place, and Britain's Max Poole from Team DSM-Firmenich PostNL, who finished third.
"It's been a while since I've been in a scenario like this if I'm honest and I just used my experience," Dunbar said.
"I was suffering a bit on the big climb. I just played my cards and said I'd gamble and back my finish."
When Dunbar accelerated ahead of the lead group, no one could keep up. Despite the Irishman’s small build, he successfully fended off the more powerful riders from the day's breakaway.
"It's incredible. Since the Vuelta last year, I think I’ve had seven or eight crashes. And of course, physically that takes its toll, but also mentally," Dunbar added.
"I thought numerous times that I might not have a future in the sport, because of the crashes and the injuries I’ve had."
Australia's Ben O'Connor, the overall leader, had a challenging day, finishing in 42nd place. Slovenia's Primoz Roglic cut his rival's lead by 37 seconds, reducing the gap to three minutes and 16 seconds.
"It was a steep finish and there’s quite a few guys that were super strong today. It’s not the worst-case scenario," O'Connor said.
"I’ve had better days and that’s the story of La Vuelta for myself so far, good day, average day, good day, average day... Hopefully, I can change that and be good every day."