Mbappe has reportedly emerged as a target for LaLiga giants Madrid, who are still looking to fill the void left by Cristiano Ronaldo after his move to Juventus last year.
The 20-year-old insisted he will not join Zinedine Zidane's side at the end of the season following his hat-trick in a 3-1 win against Monaco on Sunday, as PSG celebrated their sixth Ligue 1 title in seven seasons.
Asked about Mbappe – who became the youngest player to score 30 Ligue 1 goals in a single campaign – and his future post-match, head coach Tuchel told reporters: "The president [Nasser Al-Khelaifi] has always told me the same thing: there are no opportunities for other clubs to buy Kylian this summer.
"I've always been convinced that he'll always be our player. This is important to achieve all our objectives. To win, we need players like Kylian.
"We need to have Kylian on the team. He's a decisive player for us, he can improve, he shows it every week, every three days. There's nothing else to say about Kylian; I've already said everything.
"If we play for PSG, everyone expects us to win; we expect that from us, too. For that, we need the best players like Kylian in attack. If he plays, everyone is confident of scoring anytime."
"I was very, very happy today after Lille's draw against Toulouse because after that we were sure to be champions," added Tuchel, who replaced Unai Emery prior to the start of the season.
"It was a good feeling. Everyone expected us to be champions sooner but when you are finally champions, it's different. This is my first championship and it's really special.
"For that, I say thank you to all the staff and players because without a player, it is not possible to win trophies as a coach."