Jaco Van Zyl finished alone in third at 15-under-par, followed by Tjaart van der Walt, who came in a shot further back.
'Scott played really well. He kept coming back at me and he holed a lot of putts. He only made one mistake, and that was at the seventh,' said Goosen, the reigning U.S. Open champion.
Drummond made a poor club choice and paid for it with a double-bogey at No. 7, while Goosen birdied the same hole to go one ahead. He extended his lead to two with 40-foot swinging birdie at the par-3 13th.
Drummond, who shot a 69 in the final round, rebounded with a 60-foot birdie of his own at the 14th, but Goosen matched his birdie at the hole and birdied No. 17 to grab the three-shot edge he would carry on to victory.
'The 17th was a key birdie because it was nice to have a three-shot lead playing the last, which is a par-5. It was good just to need a two-putt for the win,' Goosen said.
This has been an incredible run for Goosen. He won last week's Johnnie Walker Classic on the European Tour after capturing last season's Order of Merit (money-winning) title.
Goosen's success has spread to others, it seems. His good friend Ernie Els captured the Heineken Classic on the European Tour earlier Sunday.
'It's been going well for the two of us,' said Goosen, who won on his home soil for the first time since the 1995 South African Open. 'And I hear we're also into the final of the cricket, so it was a good week for South Africa.'
David Park took a solo fifth at 13-under-par, followed by Michael Kirk, who came in sixth at 12-under.
Defending champion Darren Clarke shared eighth at 9-under-par 279.
Full-Field scores from the Dimension Data Pro-Am