The South African team were penalised five runs and umpires ordered the ball to be changed on the third day of the Test.
Australian match referee David Boon summoned Du Plessis on Friday.
"Du Plessis is fined 50 per cent of his match fee after breaching ICC code of conduct on Friday," the ICC said.
Du Plessis, South Africa's Twenty20 captain, had faced a range of possible penalties including forfeiting his match fees and suspension from future games - he could have missed one Test, two one-day internationals or two Twenty20s.
South African vice-captain AB de Villiers on Friday denied the ball-tampering allegations.
"Honestly, we're not the team that scratches the ball," de Villiers said. "We don't cheat - it's as simple as that. I know Faffy very well; he's the last man to try anything like that."
De Villiers said du Plessis was given the task to look after the shine on the ball.
"It's not an easy job. I thought he did it very well," de Villiers said.
Pakistan are the only team previously to have been penalised five penalty runs for ball tampering - against England in The Oval Test of 2006.
Pakistan's then captain Inzamam-ul-Haq walked off in anger, prompting Australian umpire Darrell Hair to declare the Test forfeited - the only such instance in Test cricket history.