A Court of Arbitration for Sport (Cas) panel reduced the ban to four years on Monday.
Following the judgement, the 60-year-old said he would be stepping down.
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”.
The two said the payment in 2011 was made for consultancy work Platini had carried out for Blatter between 1998 and 2002, and that they had a “gentleman’s agreement” on when the balance was settled.
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.
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.
“The Cas panel was of the opinion that a severe sanction could be justified in view of the superior functions carried out by Mr Platini (Fifa vice-president and Uefa president), the absence of any repentance and the impact that this matter has had on Fifa’s reputation,” it said.