4 returned a closing 5-under 67 around Centro Nacional de Golf in Madrid to post a 20-under 268.
"When I chose your decision in the future direct from Augusta rsquo & it wouldn;t be to show up and wander around, " I wished to get that particular tournament", Rahm told reporters.
"I've been blessed to be national champion from 16 to all ages in Spain".
It was hardly the biggest stage he's succeeded on but Jon Rahm says winning in front of a home crowd in Madrid was the most hard Sunday he's ever endured.
"It's been wonderful. It's truly been the hardest Sunday I've ever had in any tournament that I've won because the crowd wanted it so much and I wanted it so much".
Rahm (23) showed he was a special talent when winning two Rolex Series events previous year - the Dubai Duty Free Irish Open and the season-ending DP World Tour Championship in Dubai. "They payed a huge part, I came for them mainly so I'm just glad I can win this one for the Spanish people".
Dunne bogeyed 14 but regained the shot with a birdie on 15 before securing second on his own with a 72nd hole birdie and a 23rd birdie over the four days.
Further back, George Coetzee enjoyed an impressive final round as he ended the day on nine under, seeing him claim fourth place ahead of four others tied for fifth.
Rahm soon found himself in the solo lead as Elvira and Dunne both missed the green and failed to get up and down on the fourth but Elvira snatched top spot with an eagle on the next, holing a downhill left-to-righter from 35 feet.
Rahm escaped the water over the seventeenth and made it into the club house in a position adopting his grandma.
The 23-year-old has now carded seven straight rounds in the 60's, which started at the second round of the Masters.
Rahm again incurred the wrath of Sky commentators, including renowned Dougie Donnelly, for a second day running, in failing to call "Fore" after a wild drive off the 10th that would lead to a chip-in birdie and take him to 18-under.
Spain's Nacho Elvira had seemed well placed to challenge Rahm, but he sent his tee shot on the short 17th into the water and paid the price with a double bogey.
While he birdied the 15th to get back to within two shots of Rahm and Elvira, there were to be no late mistakes from the eventual victor.
The Spaniard, who's now the World number four, claimed a two-shot victory after carding a closing 67 to move 20 under for the tournament.
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