The 26-year-old Jamaican international has penned a three-year contract with the Premier League club.
"I'm delighted to be joining Palace and playing in the Premier League again. It's something everyone dreams of doing," Mariappa told the club's official website.
"I want to prove I can play at this level an I'm sure that will be the same for everyone in the squad as we aim to stay in the Premier League."
Mariappa leaves Reading after one year following a move from Watford in the summer of 2012.