The 29-year-old, ranked 306 in the world, turned a four-stroke deficit into a stunning four-shot victory on home soil at the Madeira Islands Open.
"To win my first European Tour event in my own country is very special," said Santos, who grabbed five birdies in the last seven holes. "It was a sensational round, definitely the best of my career."
Swede Magnus Carlsson was second on 270 and Dane Andreas Harto third as Spaniard Carlos Del Moral, three shots clear at the start of the day, fell away to joint fourth with a 73.
Former Madeira champion Alastair Forsyth collected his best finish in exactly 12 months by grabbing two eagles in a round of 66 to finish on 273, 15 under par. However Forsyth's share of seventh place was tinged with disappointment as it meant missing out by a stroke in qualifying among the top-five to contest the Wales Open later this month.
The Paisley pro used a rescue club to knock his ball to 10 feet for his eagle at the par-5 seventh and then drilled a 3-wood, from 270 yards out, at the par-5 16th to five feet and also holed the putt.
Sandwiched between the two eagles were three straight birdies from the 10th.
"I don't think I've ever had two eagles before in the one round," he said. "I struggled early on and made a pathetic bogey at the third, but then after eagling the seventh I made a Seve [Ballesteros] up-and-down for par at eight. So I've played the last 12 holes in seven under and turned it into a really good tournament for me."
Helensburgh's Gary Orr, runner-up to Forsyth in 2008, produced four rounds in the 60s to share 11th place on 275.