Du Plessis, who has accepted the sanction, showed dissent by “occupying the crease for a period of time” after being given out lbw before leaving the field while looking at his bat and shaking his head, the ICC said in a statement on Tuesday.
It was Du Plessis' second sanction for dissent in 12 months, after being fined 15 percent of his match fee against India in a one-day international in October.
A third offence over the next seven months would result in a suspension for the 31-year-old.
South Africa failed to teach the World Twenty20 semi-finals.
ReutersFaf du Plessis has been fined 50% of his match fee for showing dissent during Monday's win over Sri Lanka.
Original source: Faf du Plessis fined for dissent