Du Plessis appealed against his punishment on Monday, questioning whether his actions changed the condition of the ball at all.
The Chair of the ICC's Code of Conduct Commission, Michael Beloff, upheld the verdict, but stopped short of increasing the sanction to an automatic one-match ban. International Cricket Council (ICC) chief executive officer Dave Richardson welcomed the decision.
"It is the duty of the ICC to ensure fair play on the cricket field," Richardson said in a statement on Wednesday.
"Although it was not picked up by the umpires at the time, when the incident came to our attention subsequently, we felt it was our responsibility to lay a charge in this case because the ICC can't let such an obvious breach of this Law pass without taking any action.
"We are pleased that both the match referee and Mr Beloff QC have agreed with our interpretation of the Laws and hope that this serves as a deterrent to all players not to engage in this sort of unfair practice in the future."
Du Plessis, 32, was last week confirmed as South Africa's new permanent captain after AB de Villiers resigned having missed three test series in a row due to an elbow injury.
- AAP