In a statement sent to news agency AFP on Tuesday, Mbappe said PSG "were informed on July 15, 2022" of his decision to not extend his contract, adding that "the only aim of the letter was to confirm what had already been spoken about previously".
That opens the door to the forward's possible exit from Parc de Princes during this summer's transfer window, the source added, confirming a report in French sports daily L'Equipe.
Mbappe's confirmation by letter comes a little over a year since he snubbed Real Madrid by signing a surprise extension to his contract to remain at the French champions.
PSG will not entertain allowing their prize asset to leave for free when his existing deal runs out.
This raises the possibility of the World Cup runner-up switching clubs if talks with the PSG hierarchy break down between now and the end of July.
However, on Tuesday, Mbappe dismissed reports saying he wanted to join Real Madrid in the close season transfer window, saying on Twitter: "I've already said that I'm going to continue next season at PSG where I'm very happy."
Mbappe's letter comes hot on the heels of Lionel Messi's departure to Inter Miami.
It came as little surprise as reports had suggested he would not exercise an option to stay until 2025.
At an awards ceremony last month where he was voted the Ligue 1 player of the season for the fourth successive time after helping Paris Saint-Germain win a record 11th French title, he was asked about his future.
"I will be here again next season" was his reply.
Mbappe has long been admired by Real, and after already agreeing a deal to sign England midfielder Jude Bellingham from Borussia Dortmund, they could step up their interest in Mbappe following striker Karim Benzema's move to Al-Ittihad in Saudi Arabia.