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