The world No 5 regained the Stonehaven Cup after beating home pair Ashley Hall and Cameron Smith at the first extra hole in a play-off after the trio had finished tied on 12-under-par at Royal Sydney.
Spieth holed putts that combined to around 50 feet on the final three holes, including a six-footer at the last to get into the play-off, as he closed with a 69.
The 23-year-old then knocked in a 12-foot birdie putt at the 18th in the sudden-death shoot-out, with Hall unable to follow him in from closer to the hole.
It’s Spieth’s first victory since winning the Dean & Deluca Invitational on the PGA Tour in May.
As consolation, Hall and Smith, who both shot closing 66s, secured spots in next year’s Open Championship at Royal Birkdale along with a third Australian, Aaron Baddeley.
Former US Open champion Geoff Ogilvy had been on course for victory until he went bogey-double bogey at the 15th and 16th. A closing 73 left him tied for fourth.
English starlet Charley Hull takes a one-shot lead into the final round of the LPGA Tour’s CME Group Tour Championship in Florida.
The 20-year-old moved to the top of the leaderboard on 13-under after a flawless 66 in the season-ending event in Naples.
Bidding for her first victory on the US circuit, she’s a shot ahead of American Brittany Lincicome and Korea’s So Yeon Ryu.
Halfway leader Lydia Ko is in a group a shot further back after 73 - 11 shots more than the Kiwi’s second-day effort.
Catriona Matthew sits joint-44th on level-par after a third-round 74 that followed a brace of 71s from the Scottish No 1.
Canadian rookie Mackenzie Hughes is on course for victory in the PGA Tour’s RSM Classic at Sea Island in Georgia.
The 25-year-old recovered from a potentially disastrous triple-bogey 7 at the 11th in the third round to repair the damage with birdies at the 13th, 15th and 16th.
A two-under-par 68 put him on 16-under, one shot ahead of Billy Horschel (65), Camilo Villegas (64) and Cheng Tsung Pan (67).
“Everyone will be fighting the same nerves,” said Hughes as he looked ahead to the final round.
Jane Turner completed a wire-to-wire win in the Ladies European Tour Qualifying School pre-qualifier in Morocco.
The Lothians player carded rounds of 65, 73, 72 and 74 for a four-under-par 284 total, winning by five shots from Welsh woman Chloe Williams.
Gabrielle Macdonald and Rachael McQueen, the two other Scots in the field, finished 35th and 53rd respectively.