The 30-year-old caught and passed Colombian Santiago Buitrago with just 300 metres left of the mountainous 210km slog from Velez-Malaga to Penas Blancas.
Buitrago struck out for home inside the final two kilometres to distance his three breakaway companions, including Dunbar, who initially did not react.
But the Pinarello Q36.5 rider launched his counterattack in the final kilometre and wasted little time in reeling in Buitrago, who is the leader of the King of the Mountains competition.
Dunbar finished 14 seconds ahead of Buitrago, with his British teammate Thomas Gloag coming home in third, 23 seconds back.
Mas comfortably held onto his one-minute 37-second lead over four-time former champion Primoz Roglic as the overall contenders crossed the line together.
Roglic's Red Bull-Bora Hansgrohe team and the EF Education-Easypost outfit of former Giro d'Italia winner Richard Carapaz tried to increase the pace on the 18km final climb to the finish.
But they were unable to exert any pressure on Mas, who followed comfortably as he bids for a first Vuelta title after three previous second-place finishes.