The 25-year-old had been locked in talks with Royals boss Brian McDermott over the past couple of days after Watford accepted an undisclosed bid for the defender at the weekend.
Following those discussions, Mariappa has opted to pen a four-year deal at the Madejski Stadium.
The London-born centre-back has become the sixth player to join Reading this summer.
Restore Data
| Team | P | W | D | L | F | A | GD | PTS | |
| 1 | Wycombe WanderersWycombe | 15 | 11 | 2 | 2 | 35 | 18 | 17 | 35 |
| 2 | Wrexham | 16 | 9 | 4 | 3 | 25 | 11 | 14 | 31 |
| 3 | Stockport CountyStockport | 17 | 8 | 6 | 3 | 29 | 17 | 12 | 30 |
| 4 | Birmingham CityBirmingham | 14 | 9 | 3 | 2 | 26 | 15 | 11 | 30 |
| 5 | Huddersfield TownHuddersfield | 15 | 8 | 2 | 5 | 23 | 15 | 8 | 26 |
| 6 | Barnsley | 16 | 7 | 5 | 4 | 24 | 20 | 4 | 26 |
| 7 | Lincoln CityLincoln | 16 | 7 | 5 | 4 | 23 | 20 | 3 | 26 |
| 8 | Reading | 15 | 8 | 2 | 5 | 24 | 22 | 2 | 26 |
| 9 | Bolton WanderersBolton | 15 | 8 | 2 | 5 | 24 | 25 | -1 | 26 |
| 10 | Mansfield TownMansfield | 14 | 7 | 3 | 4 | 20 | 16 | 4 | 24 |
| 11 | Exeter CityExeter | 15 | 7 | 2 | 6 | 14 | 13 | 1 | 23 |
| 12 | Peterborough UnitedPeterborough | 16 | 6 | 3 | 7 | 32 | 29 | 3 | 21 |
| 13 | Bristol Rovers | 16 | 6 | 3 | 7 | 17 | 21 | -4 | 21 |
| 14 | Charlton AthleticCharlton | 15 | 5 | 4 | 6 | 16 | 17 | -1 | 19 |
| 15 | Stevenage | 16 | 5 | 4 | 7 | 11 | 16 | -5 | 19 |
| 16 | Northampton TownNorthampton | 16 | 4 | 6 | 6 | 18 | 21 | -3 | 18 |
| 17 | Wigan AthleticWigan | 15 | 4 | 5 | 6 | 13 | 12 | 1 | 17 |
| 18 | Rotherham UnitedRotherham | 16 | 4 | 5 | 7 | 14 | 19 | -5 | 17 |
| 19 | Blackpool | 16 | 4 | 5 | 7 | 22 | 30 | -8 | 17 |
| 20 | Crawley TownCrawley | 17 | 4 | 4 | 9 | 15 | 27 | -12 | 16 |
| 21 | Leyton Orient | 15 | 4 | 3 | 8 | 14 | 18 | -4 | 15 |
| 22 | Cambridge UnitedCambridge | 15 | 3 | 3 | 9 | 13 | 24 | -11 | 12 |
| 23 | Shrewsbury TownShrewsbury | 16 | 3 | 2 | 11 | 16 | 29 | -13 | 11 |
| 24 | Burton Albion | 15 | 1 | 5 | 9 | 15 | 28 | -13 | 8 |
| > League One Full Table | |||||||||
