Andrew McLardy (70) and Gary Orr (69) shared second place at 11-under-par 277. David Higgins finished alone in fourth place at 10-under-par 278.
Derksen came out firing in the final round. He drained a birdie putt at the par-4 second and followed with a birdie at the third to get to minus-10.
The 31-year-old parred his next six holes before catching fire on the back nine. Derksen dropped in a birdie putt at the 10th and made it two in a row with a birdie at 11.
Derksen, whose first European Tour win came at the 2003 Dubai Desert Classic, stumbled to his lone final-round bogey at the par-4 13th. He reclaimed that lost stroke with a birdie at the par-3 15th at Santo da Serra Golf Club.
The Dutchman came to the 18th tee tied for the lead with Orr. Derksen then closed the tournament in style as he birdied the 18th from 15 feet out.
'Obviously I am delighted to have won,' said Derksen. 'My birdie on the 18th was excellent and really won the tournament for me because when I was playing the hole I was level with Gary and didn't think he would finish with a bogey, so it was a great birdie.'
McLardy wrapped bogeys at five and seven around a birdie at the sixth. He dropped another stroke at the ninth. However, the South African birdied 10 and 11 to get back to minus-9.
The 31-year-old birdied the 15th and 18th to share second place.
Orr parred his first seven holes before converting his first birdie try at the eighth. Around the turn, the Scotsman birdied 11 and 12 to get to 11 under.
The 34-year-old drained his final birdie at the 14th. Orr had a chance to force a playoff, but needed to birdie the last. However, he came up short of the 18th green and walked away with a bogey to finish two back.
Tom Whitehouse took fifth place at 8-under-par 280. John Bickerton finished one stroke further back at minus-7.
Kyron Sullivan, who held at least a share of the lead after each of the first three rounds, struggled to a final-round 75. He finished in a tie for seventh at 6-under-par 282 with Simon Wakefield, Andrew Coltart and Mikko Ilonen.