The Frenchman's ban has been lowered from six to four years, although this is still likely to signal the end of the 60-year-old's career in the sport.
Platini and former FIFA counterpart Sepp Blatter, were last year found guilty of ethics breaches over a 2m Swiss Franc (£1.3m) “disloyal payment”.
The pair, who deny wrongdoing, had their original bans reduced from eight to six years by the FIFA Appeals Committee.
Platini had taken his case to CAS seeking to get the ban overturned, but a three-man panel said it “was not convinced by the legitimacy of the payment”.
After CAS returned its judgement, a statement from Platini’s lawyers said: “Michel Platini announces that he will resign as president of UEFA at the organisation’s next congress.”
It added the Frenchman was “deeply disappointed” by the decision, which he considered “an injustice”.
CAS also said an 80,000 Swiss Franc (£57,200) fine should be lowered to 60,000 (£42,900). However, the panel was damning of Platini, saying his “behaviour was not ethical or loyal”.
It found his dealings with Blatter had breached the ethics code of world governing body FIFA, and while his six-year ban was “too severe”, it decided a four-year punishment should be handed down – the equivalent to a presidential term in office.