After confirming the arrival of right-back Jack Hunt from Huddersfield Town earlier on Monday, the Eagles have now swooped for the 26-year-old, who has put pen to paper on a three-year deal for an undisclosed fee.
Mariappa only joined the Royals last summer from Watford after their promotion to the Premier League and will be hoping to help his new employers avoid a similar fate to that of Nigel Adkins's side, who went straight back down to the Championship.
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"Playing there last season was a great challenge as it is 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 an I'm sure that will be the same for everyone in the squad as we aim to stay in the Premier League."
As revealed by Goal, Palace are also expected to tie up a deal for Arsenal striker Nicklas Bendtner before the deadline.