Steyn swung the ball at pace to rip out three quick wickets and record figures of 4/35 as Australia were limited to 217 for nine.
Du Plessis then fell four runs short of what would have been his fourth hundred of the series as the Proteas chased down their target with 55 balls to spare in front of a packed Harare Sports Club.
The South African quick made the early breakthrough when he had Phil Hughes brilliantly caught by AB de Villiers, but his most significant contribution came when he swung the ball at pace to pick up three wickets in the middle period.
"I think they bowled very, very well," Australia captain George Bailey said. "It's actually easier to swallow when you can say that you were outplayed.
"I thought they bowled really tight and made it hard for us to score early, and then once they got the ball reversing, Dale Steyn was able to show his class."
Brad Haddin was also trapped in front at the start of Steyn's next over, and with Wayne Parnell and Morne Morkel also striking Australia were reduced to 144 for eight in the 38th over.
"He's the best bowler of reverse swing in the world by a long way," South Africa captain AB de Villiers said of Steyn.
"I think he showed more skills than most of our bowlers and the Australian bowlers.
