Nottingham Forest right-back Chris Gunter the later became the club's second signing of the day
They are the club's fifth and sixth major signings of the summer for newly-promoted Reading, following the arrivals at the Madejski Stadium of Pavel Pogrebnyak, Danny Guthrie, Nicky Shorey and Garath McCleary.
Mariappa, 25, played briefly in the top flight for Reading in 2005/06.
Watford have lined up Fitz Hall to replace Mariappa, after the defender was released by QPR at the end of last season.
The Hornets are also believed to be in talks over the eye-capturing acquisition of former Italy striker Filippo Inzaghi.
The prolific 38-year-old former Azzurri team-mate Gianfranco Zola recently took over as manager at Vicarage Road.
Former Cardiff and Tottenham full-back Gunter joins from Forest for an undisclosed fee, although Reading say they have made a "sizeable outlay" for the 22-year-old.
Gunter has penned a three-year contract at the Madejski Stadium, with the option for a further year.
"Having spoken to the manager, [director of football] Nick Hammond and now the chairman, everyone has made me feel really welcome and I'm really looking forward to this opportunity," Gunter said.
"If you form a team spirit, a togetherness, you will always do well in football - and if you work hard you will get your rewards. Reading have certainly done that in the last few years. Hopefully that can carry on. Right now, I need to focus on impressing everyone at this club and I'm really excited to get going."