Fernandrez-Castano closed with a 7-under 65 to get into the clubhouse first at 16-under-par 200. He was joined there by Markus Brier, who carded a 4-under 68 in the final round at Castello di Tolcinasco Golf & Country Club.
In the playoff, Fernandez-Castano found a fairway bunker off the 18th tee before Brier's tee ball trickled into the first cut of rough. Brier found the green with his second shot, but had over 30 feet for birdie.
Fernandez-Castano was on the back fringe and a similar distance from the hole as Brier. The Austrian putted first and his birdie effort lipped out. Fernandez-Castano's birdie putt missed as well and it was back to the 18th tee.
The Spaniard knocked his tee shot into the right rough, while Brier landed in the fairway. Brier's approach came to rest 15 feet from the hole, but Fernandez-Castano dropped his shot within 8 feet.
Brier's birdie putt rolled by the hole, then he watched as Fernandez-Castano sank his birdie putt for the win.
'It is a good tournament with a lot of tradition, I can't be happier,' Fernandez-Castano exclaimed.
Second-round leader Nick Dougherty struggled down the stretch as he shot 2- over par over the final nine holes. He had a 12-footer for birdie at the last to get into the playoff, but he missed on the high side.
The Englishman carded a 2-under 70 to share third place with 2006 champion Francesco Molinari (65), Henrik Nystrom (66) and Fredrik Andersson (64).
The event was shortened to 54 holes after lengthy weather delays Friday and Saturday. Sunday's third and final round was delayed at the outset due to fog.
Fernandez-Castano opened with a birdie on the first, but parred the next eight holes to turn in minus-10. Around the turn, he ran off three straight birdies from the 10th.
The 26-year-old stumbled to a bogey on 13, but erased that mistake with a birdie on 14. Fernandez-Castano came right back with an eagle on the par-5 15th to get to 15-under.
Fernandez-Castano nearly holed his approach at the 17th. He tapped in a birdie putt from within 6 inches to join Dougherty in the lead. The Spaniard two-putted for par from 35 feet out at the last to end at 16 under.
'I knew I had to play very well today, but five shots, especially on a course like this that everybody can make lots of birdies, I never thought that 65 was going to be good enough,' said Fernandez-Castano. 'I thought the winning score was going to be closer to 18 under.'
Brier birdied the first, then holed out for birdie at the second. After a birdie at five, he struggled to a bogey on six. He parred the next nine holes to remain at 14 under.
The Austrian poured in back-to-back birdies from the 16th to grab a piece of the lead. Brier two-putted for par from 18 feet to join Fernandez-Castano in the clubhouse at 16 under.
Dougherty looked to be running away from the field on the front nine. He eagled the first and birdied the third to move one clear of Brier at 16 under.
The Englishman birdied the ninth for the third consecutive round to move to 17 under, three strokes clear of eight players. Dougherty dropped a shot on 12 and bogeyed the 17th to fall out of the lead for the first time all day.
Raphael Jacquelin closed with a 7-under 65 Sunday to end in a tie for seventh at 14-under-par 202. He was joined there by Marcus Fraser, Andrew McLardy and Alexandre Rocha.