Mateta’s arrival at Selhurst Park will augment a Crystal Palace forward line that has struggled for goals this season.
The attacking trio of Christian Benteke, Jordan Ayew and Michy Batshuayi have scored just four Premier League goals for this season for Palace, who lie 13th on the Premier League table.
Mateta scored 10 goals in 17 appearances for Mainz in the first part of the 2020/2021 season.
"I'm very happy to come to Crystal Palace and I look forward to training," the 23-year-old Frenchman told the club's website.
"Playing in the Premier League was my dream when I was a child. Now I can play, I will show the fans of Crystal Palace what I can do."
"We have followed Jean-Philippe's development with keen interest for some time, and I'm so pleased that we have been able to bring him to south London," added Palace chairman Steve Parish.
"He is a player that is full of potential with a proven goal-scoring record, and his signing reiterates our commitment to ensuring we have a well-balanced squad in terms of youth and experience for the seasons ahead."
Mateta could make his debut for Palace against West Ham United in the Premier League on Tuesday.
