The 27-year-old Spaniard putted from just off the green on the 18th hole at the Belfry to within an inch, and then watched as defending champion Westwood chipped poorly and missed a 15-foot par putt. Both players parred the 18th on the first two playoff holes.
The victory earned Fernandez-Castano $300,000. He celebrated by taking a dip in the lake that fronts the final green and came to his media center interview clad only in two towels.
It was cold but not that cold, but it smelled terrible'like I do right now, he said.
It was his fourth European Tour victory and third in a playoff.
This was probably the best of the four. It has been a great day, playing with Lee and Michael Campbell, he said. Now I think I have qualified for the last Volvo Masters at Valderrama this year. That was one of my targets.
Fernandez-Castano began the final round three strokes behind Westwood and Campbell, but shot a 5-under 67. He finished at 12-under 276, along with Westwood.
Westwood had a chance to win it on the last hole, but missed a 20-foot birdie attempt.
Campbell, the 2005 U.S. Open champion, shot 72 in the final round after bogeys at the 16th and 18th holes to take third place, his fourth top-10 finish in five events after missing 10 cuts this year.
Mikael Lundgren of Sweden, one of the first-round leaders, was fourth after a 70.
Westwood and Fernandez-Castano were level with five holes to play and matched each other stroke for stroke the rest of the way, although the Spaniard needed a fine chip from deep rough to within 3 feet on the 18th to make par.
Earlier, he chipped in at the short 12th after hitting his tee shot into the water and taking a drop.
I struggled with my swing all day and didnt feel in control at any point, said Westwood, adding that he was still tired from playing in the Ryder Cup last weekend. I couldnt feel my swing. I hit a lot of poor iron shots, a lot of sloppy shots.