Fernandez-Castano began the final round one shot behind third-round leader Garth Mulroy and finally grabbed the lead on the 17th hole when Mulroy bogeyed after a poor second shot.
The Spaniard then birdied the final hole and Mulroy settled for a par, giving Fernandez-Castano a two-shot victory at 24-under 264.
It marked his sixth career win on the European Tour -- he also took the Italian event in 2007.
Sharing third place at minus-20 were Italian Matteo Manassero and Gregory Bourdy, each of whom fired 7-under 65s.
Tied for fifth were Martin Kaymer (67) and Nicolas Colsaerts (67).