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 | 19 | 13 | 4 | 2 | 43 | 22 | 21 | 43 |
| 2 | Birmingham CityBirmingham | 18 | 13 | 3 | 2 | 34 | 16 | 18 | 42 |
| 3 | Wrexham | 20 | 12 | 5 | 3 | 30 | 13 | 17 | 41 |
| 4 | Huddersfield TownHuddersfield | 19 | 11 | 3 | 5 | 30 | 18 | 12 | 36 |
| 5 | Stockport CountyStockport | 20 | 9 | 6 | 5 | 32 | 21 | 11 | 33 |
| 6 | Reading | 19 | 9 | 4 | 6 | 30 | 28 | 2 | 31 |
| 7 | Barnsley | 20 | 8 | 6 | 6 | 29 | 26 | 3 | 30 |
| 8 | Bolton WanderersBolton | 18 | 9 | 3 | 6 | 28 | 29 | -1 | 30 |
| 9 | Lincoln CityLincoln | 20 | 7 | 7 | 6 | 26 | 25 | 1 | 28 |
| 10 | Blackpool | 19 | 7 | 5 | 7 | 29 | 31 | -2 | 26 |
| 11 | Mansfield TownMansfield | 18 | 7 | 4 | 7 | 22 | 22 | 0 | 25 |
| 12 | Stevenage | 18 | 7 | 4 | 7 | 15 | 17 | -2 | 25 |
| 13 | Peterborough UnitedPeterborough | 19 | 7 | 3 | 9 | 37 | 35 | 2 | 24 |
| 14 | Charlton AthleticCharlton | 19 | 6 | 6 | 7 | 18 | 19 | -1 | 24 |
| 15 | Exeter CityExeter | 19 | 7 | 3 | 9 | 17 | 21 | -4 | 24 |
| 16 | Wigan AthleticWigan | 19 | 6 | 5 | 8 | 17 | 16 | 1 | 23 |
| 17 | Rotherham UnitedRotherham | 18 | 6 | 5 | 7 | 19 | 20 | -1 | 23 |
| 18 | Leyton Orient | 19 | 6 | 4 | 9 | 19 | 20 | -1 | 22 |
| 19 | Northampton TownNorthampton | 20 | 5 | 6 | 9 | 21 | 29 | -8 | 21 |
| 20 | Bristol Rovers | 19 | 6 | 3 | 10 | 17 | 28 | -11 | 21 |
| 21 | Crawley TownCrawley | 19 | 5 | 4 | 10 | 20 | 32 | -12 | 19 |
| 22 | Cambridge UnitedCambridge | 19 | 4 | 5 | 10 | 20 | 31 | -11 | 17 |
| 23 | Burton Albion | 19 | 2 | 6 | 11 | 16 | 30 | -14 | 12 |
| 24 | Shrewsbury TownShrewsbury | 19 | 3 | 2 | 14 | 19 | 39 | -20 | 11 |
| > League One Full Table | |||||||||
