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 | Birmingham CityBirmingham | 28 | 20 | 6 | 2 | 50 | 19 | 31 | 66 |
| 2 | Wycombe WanderersWycombe | 29 | 17 | 8 | 4 | 55 | 30 | 25 | 59 |
| 3 | Wrexham | 29 | 16 | 7 | 6 | 43 | 25 | 18 | 55 |
| 4 | Stockport CountyStockport | 31 | 15 | 9 | 7 | 47 | 30 | 17 | 54 |
| 5 | Charlton AthleticCharlton | 30 | 14 | 8 | 8 | 40 | 29 | 11 | 50 |
| 6 | Huddersfield TownHuddersfield | 29 | 14 | 7 | 8 | 41 | 26 | 15 | 49 |
| 7 | Leyton Orient | 29 | 14 | 5 | 10 | 43 | 25 | 18 | 47 |
| 8 | Bolton WanderersBolton | 30 | 14 | 5 | 11 | 47 | 46 | 1 | 47 |
| 9 | Reading | 30 | 13 | 7 | 10 | 44 | 42 | 2 | 46 |
| 10 | Barnsley | 30 | 12 | 7 | 11 | 42 | 41 | 1 | 43 |
| 11 | Blackpool | 30 | 9 | 13 | 8 | 42 | 41 | 1 | 40 |
| 12 | Lincoln CityLincoln | 30 | 10 | 10 | 10 | 36 | 35 | 1 | 40 |
| 13 | Stevenage | 29 | 11 | 7 | 11 | 28 | 30 | -2 | 40 |
| 14 | Rotherham UnitedRotherham | 30 | 10 | 8 | 12 | 34 | 35 | -1 | 38 |
| 15 | Mansfield TownMansfield | 29 | 11 | 4 | 14 | 35 | 38 | -3 | 37 |
| 16 | Wigan AthleticWigan | 28 | 9 | 7 | 12 | 27 | 28 | -1 | 34 |
| 17 | Northampton TownNorthampton | 30 | 8 | 9 | 13 | 29 | 45 | -16 | 33 |
| 18 | Exeter CityExeter | 29 | 9 | 5 | 15 | 33 | 46 | -13 | 32 |
| 19 | Bristol Rovers | 29 | 9 | 5 | 15 | 29 | 45 | -16 | 32 |
| 20 | Peterborough UnitedPeterborough | 30 | 8 | 6 | 16 | 46 | 57 | -11 | 30 |
| 21 | Shrewsbury TownShrewsbury | 30 | 7 | 6 | 17 | 31 | 50 | -19 | 27 |
| 22 | Crawley TownCrawley | 29 | 7 | 6 | 16 | 33 | 53 | -20 | 27 |
| 23 | Burton Albion | 30 | 5 | 11 | 14 | 31 | 45 | -14 | 26 |
| 24 | Cambridge UnitedCambridge | 30 | 5 | 8 | 17 | 30 | 55 | -25 | 23 |
| > League One Full Table | |||||||||
