Mariappa, who has not played for Reading in the Championship this season, has penned a three-year deal with Ian Holloway's side.
The Jamaica international, 26, did play in Reading's shock 6-0 loss to Peterborough United in the League Cup last week but was dropped for Saturday's visit to Yeovil Town.
And the centre-back, who made 29 Premier League appearances for Reading last term, has now joined Palace as they bid to retain their top-flight berth this season.
"I'm delighted to be joining Palace and playing in the Premier League again," Mariappa told Palace's official website. "It's something everyone dreams of doing.
"Playing there last season was a great challenge as it is the biggest league in the world and you're playing against the best players, so when this opportunity came to me I jumped at it.
"Selhurst Park is always a great place to play, the atmosphere has always been amazing and I've had some great games against this club.
"I want to prove I can play at this level and I'm sure that will be the same for everyone in the squad as we aim to stay in the Premier League."
Mariappa made his debut for Watford in May 2005 and went on to feature for them 247 times before joining Reading in July last year.
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