The Court of Arbitration for Sport (CAS), where Platini filed an appeal against his ban in February, reduced his suspension from six years to four, saying the penalty initially imposed by FIFA's ethics committee was "too severe."
But the court said it was "not convinced" that the USD 2.0 million (1.8 million euros) payment Platini received from FIFA in 2011 was legitimate.
The payment was ordered by FIFA's disgraced ex-president Sepp Blatter, who was also brought down over the infamous transaction.
Platini, once the front-runner to succeed Blatter at FIFA and become the most powerful man in the sport, said in a statement that following the ruling he had no choice but to resign from UEFA.
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"As agreed with the national associations, I resign as president of UEFA to pursue my fight before the Swiss courts to prove my probity in this case.
He called the CAS ruling "a profound injustice."
UEFA's executive committee is to meet in Basel on May 18, before the Europa League final, when it will likely begin the process of selecting Platini's replacement, although an election could be put off for several more weeks.
Platini and Blatter have both denied any wrongdoing. They have insisted the USD 2.0 million was part of a legitimate oral contract in exchange for consulting work performed by Platini.