Anders Hansen posted a 66 to take second place at 16-under-par 268. Terry Price was one shot further back at 15-under-par 269, while Paul Broadhurst and Raphael Jacquelin shared fourth place at 14-under-par 270.
Sterne was on the outside looking in heading into Club de Campo this week needing a strong finish to retain his card for 2005, and he did just that.
'The whole year has been a tough year, especially the last three weeks,' said Sterne. 'I knew I had to perform. I wanted to give myself a chance going into the back nine, and I did and I pulled it off.'
Sterne parred his first three holes before an eagle at the par-5 fourth lifted him to 14-under. He continued to build momentum with a birdie at the very next hole and added a birdie at the par-5 seventh.
The 23-year-old tallied a birdie at the par-3 ninth to make the turn at minus-17, but was in trouble at the par-3 11th after his tee shot missed the putting surface. Sterne played his second to 20 feet and drained the par putt to remain mistake-free on Sunday.
While his challengers were scrambling on the inward half, Sterne ran home an 18-foot putt for a birdie at the par-5 14th to leave little in doubt. Sterne then parred his way into the clubhouse to complete the breakthrough win.
'I'm excited,' said the South African. 'Obviously a new experience for me. I've won a lot of amateur tournaments and I just tried to do the same thing here. I'm looking forward to next year.'
Hansen jumped out of the gate with a birdie at the opening hole and added birdies at the fourth and seventh to move to 14 under around the turn.
The Dane stumbled to a bogey at the 11th, but recovered with back-to-back birdies starting at the par-4 13th. Hansen then birdied the par-4 16th to secure a runner-up finish.
While Sterne earned his playing card for next year and Price moved into the top 60 on the Order of Merit to gain a spot in next week's Volvo Masters at Valderrama, there was plenty disappointment to go around.
Ivo Giner needed a second-place finish to maintain his card, but a 1-under 70 was only good enough for a share of sixth at 13-under-par 271 along with Wade Ormsby and Darren Fichardt.
Johan Edfors also needed at least a runner-up finish to hold onto his playing status on the European Tour. He stood atop the leaderboard on Saturday until his ball disappeared after an errant tee shot on the 18th, and Edfors managed only a round of 70 to finish six shots off the pace.
Edfors was joined by Trevor Immelman, Henrik Stenson, Brett Rumford and Raymond Russell at 12-under-par 272.