Aurelien Tchouameni has signed a six-year deal with Real Madrid.
The 22-year-old joined Real Madrid in an initial €80 million deal which could go up to €100m with add-ons.
He has been brought as a direct replacement to Casemiro who recently signed a contract extension with the La Liga champions.
Mbappe who decided to stay put at PSG wanted Tchouameni to join him at the French club.
“Kylian decided to stay at PSG and he knew I was leaving Monaco,” Tchouameni said at his presentation on Tuesday.
“He wanted to know if I would join him at PSG. But my first choice was Real Madrid and he understood that completely.”
The Frenchman explained why he chose Real Madrid despite interest from Liverpool and PSG.
“I had the chance to choose other teams but once I knew Real Madrid were interested, I did not have a single doubt,” he added.
“I spoke with other clubs but in my head I was always waiting for Madrid. I spoke with my agent, my family and [Monaco] to find the best agreement for everyone.
“Madrid are the biggest club in the world, they have a team with incredible players and I think it’s the best decision for me.”