There was no change at the top of the general classification despite a brave solo attack from Thibaut Pinot, meaning Briton Simon Yates retains the red jersey.
He holds a one-second lead over home favourite Alejandro Valverde and a 14-second advantage over Colombian Nairo Quintana.
Frenchman Pinot broke free from the general classification group as part of a 19-rider contingent around the midway point of the 207.8km route from Mombuey to Ribeira Sacra. Luintra, the longest stage of the 21-day race.
He threatened to prise the red jersey from Yates, but could not last the distance, losing energy in the final few kilometres to finish 10th, one minute 50 seconds behind De Marchi.
He gained a measly 12 seconds on the general classification group which leaves him 16th overall.
De Marchi battled it out with Jhonatan Restrepo in heavy rain on the final climb, finally dropping the Colombian inside the last four kilometres and crossing the finish line 28 seconds ahead of him.
It was his first solo grand tour stage win since his 2015 victory at the Vuelta, although he won a team time trial last year.
"If I waited for the sprint for sure I'd have finished second," said De Marchi, who is known for breakaways and was named the most combative rider at the 2014 Tour de France.
"The only option I had was to try everything on the last climb. Honestly, I didn't have my best legs, but it was just in the head. At one moment today I just said that I just had to keep trying because it was a real battle."
Pinot, meanwhile, said he had no regrets about his courageous but ultimately futile attempt to make an impact on the general classification.
"It was a good strategy. It might have worked out, it might not have done, but I wanted to have fun and race aggressively," said the Frenchman.
Thursday's stage is a 181.1km ride from Mondonedo to Faro de Estaca de Bares Manon.
(Reporting by Richard Martin; Editing by Toby Davis)
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