The England defender turned down a new contract offer from Bayern Munich to join Monaco in a free transfer this summer.
Presented to the local media on Thursday, former Tottenham centre-half Dier began: "My contract with Bayern Munich had come to an end and I had the ambition to take on a new challenge. Joining Monaco was the best decision for me and my family, both personally and sportingly.
"The first discussions with Thiago Scuro and Carlos Aviña were very positive, and I was immediately convinced by the project they presented to me."
On Monaco, Dier also said: "Monaco brings back many memories for me. I remember facing the team in the 2016-2017 season, when I played for Tottenham. That year, the club managed to reach the semi-finals of the competition. Of course, I also remember the Champions League final played by the Monegasques in 2004. They faced Porto, and I was still living in Portugal at the time, so I had heard about it."
Competitive Ligue 1
Now 31, Dier is confident adjusting to the demands of Ligue 1.
He added: "I have watched a lot of Ligue 1 games, especially since I started talking to the management. It is a very competitive league, the competition for qualification for the Champions League was tough last season.
"In addition, Paris Saint-Germain's final victory in Europe helps to strengthen the image of French football."