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 | 23 | 16 | 5 | 2 | 41 | 17 | 24 | 53 |
| 2 | Wycombe WanderersWycombe | 24 | 15 | 6 | 3 | 50 | 26 | 24 | 51 |
| 3 | Wrexham | 25 | 15 | 6 | 4 | 37 | 18 | 19 | 51 |
| 4 | Huddersfield TownHuddersfield | 23 | 13 | 5 | 5 | 36 | 19 | 17 | 44 |
| 5 | Barnsley | 25 | 12 | 6 | 7 | 39 | 33 | 6 | 42 |
| 6 | Reading | 24 | 12 | 5 | 7 | 39 | 33 | 6 | 41 |
| 7 | Stockport CountyStockport | 25 | 10 | 8 | 7 | 37 | 27 | 10 | 38 |
| 8 | Leyton Orient | 24 | 11 | 4 | 9 | 32 | 22 | 10 | 37 |
| 9 | Mansfield TownMansfield | 23 | 11 | 4 | 8 | 31 | 26 | 5 | 37 |
| 10 | Bolton WanderersBolton | 23 | 11 | 4 | 8 | 35 | 34 | 1 | 37 |
| 11 | Charlton AthleticCharlton | 23 | 9 | 7 | 7 | 27 | 21 | 6 | 34 |
| 12 | Lincoln CityLincoln | 25 | 8 | 8 | 9 | 28 | 30 | -2 | 32 |
| 13 | Stevenage | 23 | 8 | 7 | 8 | 18 | 20 | -2 | 31 |
| 14 | Exeter CityExeter | 24 | 9 | 4 | 11 | 28 | 31 | -3 | 31 |
| 15 | Blackpool | 24 | 7 | 9 | 8 | 32 | 35 | -3 | 30 |
| 16 | Rotherham UnitedRotherham | 23 | 7 | 7 | 9 | 21 | 23 | -2 | 28 |
| 17 | Wigan AthleticWigan | 23 | 7 | 6 | 10 | 21 | 23 | -2 | 27 |
| 18 | Northampton TownNorthampton | 25 | 6 | 8 | 11 | 24 | 39 | -15 | 26 |
| 19 | Peterborough UnitedPeterborough | 24 | 7 | 4 | 13 | 41 | 46 | -5 | 25 |
| 20 | Bristol Rovers | 24 | 7 | 4 | 13 | 22 | 38 | -16 | 25 |
| 21 | Crawley TownCrawley | 23 | 5 | 5 | 13 | 24 | 43 | -19 | 20 |
| 22 | Shrewsbury TownShrewsbury | 24 | 4 | 5 | 15 | 24 | 44 | -20 | 17 |
| 23 | Cambridge UnitedCambridge | 24 | 4 | 5 | 15 | 22 | 43 | -21 | 17 |
| 24 | Burton Albion | 24 | 2 | 8 | 14 | 20 | 38 | -18 | 14 |
| > League One Full Table | |||||||||
