The Premier League newboys have added the highly-rated defender to their ranks, with the 25-year-old becoming Brian McDermott's sixth signing of a busy summer
The 25-year-old defender, who made 247appearances during a seven-year spell at Watford, has penned a three-year contract.
And Mariappa revealed he is relishing the opportunity to play in the Premier League, telling the club's official website: "It's a great feeling to have signed.
"The Premier League is the best in the world, I'm delighted to have the opportunity to come here, try to work my way into the team and perform for Reading Football Club."
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"I'm delighted to get the deal to sign Aidy over the line, we have been chasing him all summer,” he said.
“He's been a very important player for Watford over a number of years, he is well known to most of our players already and also the staff here.
“Every single person I have spoken to about him has nothing but good things to say about him both as a player and as a person."
Club chairman Sir John Madejski believes Mariappa’s arrival is great news for the club, stating: "I would like to welcome Adrian to Reading Football Club, this is another very positive step by the club and one I am delighted has been completed.
“Adrian is very highly thought of and we hope he can develop even further under Brian McDermott and his staff."
Mariappa is the sixth summer arrival at the club, following the signings of Nicky Shorey, Pierce Sweeney, Gareth McCleary, Danny Guthrie and Pavel Pogrebnyak.