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 | 36 | 25 | 8 | 3 | 59 | 24 | 35 | 83 |
| 2 | Wrexham | 38 | 22 | 8 | 8 | 52 | 30 | 22 | 74 |
| 3 | Wycombe WanderersWycombe | 37 | 20 | 11 | 6 | 64 | 36 | 28 | 71 |
| 4 | Charlton AthleticCharlton | 38 | 19 | 9 | 10 | 49 | 35 | 14 | 66 |
| 5 | Stockport CountyStockport | 38 | 18 | 11 | 9 | 53 | 35 | 18 | 65 |
| 6 | Huddersfield TownHuddersfield | 37 | 18 | 7 | 12 | 52 | 34 | 18 | 61 |
| 7 | Bolton WanderersBolton | 37 | 18 | 6 | 13 | 59 | 55 | 4 | 60 |
| 8 | Leyton Orient | 38 | 18 | 5 | 15 | 54 | 38 | 16 | 59 |
| 9 | Reading | 37 | 16 | 11 | 10 | 52 | 46 | 6 | 59 |
| 10 | Blackpool | 38 | 13 | 15 | 10 | 57 | 50 | 7 | 54 |
| 11 | Barnsley | 38 | 15 | 8 | 15 | 52 | 54 | -2 | 53 |
| 12 | Lincoln CityLincoln | 38 | 13 | 11 | 14 | 53 | 45 | 8 | 50 |
| 13 | Stevenage | 38 | 13 | 10 | 15 | 35 | 39 | -4 | 49 |
| 14 | Rotherham UnitedRotherham | 37 | 12 | 9 | 16 | 42 | 45 | -3 | 45 |
| 15 | Peterborough UnitedPeterborough | 37 | 12 | 9 | 16 | 56 | 60 | -4 | 45 |
| 16 | Exeter CityExeter | 37 | 12 | 9 | 16 | 40 | 51 | -11 | 45 |
| 17 | Mansfield TownMansfield | 37 | 12 | 8 | 17 | 43 | 51 | -8 | 44 |
| 18 | Wigan AthleticWigan | 36 | 11 | 10 | 15 | 33 | 36 | -3 | 43 |
| 19 | Northampton TownNorthampton | 38 | 10 | 12 | 16 | 37 | 56 | -19 | 42 |
| 20 | Bristol Rovers | 38 | 12 | 6 | 20 | 39 | 61 | -22 | 42 |
| 21 | Burton Albion | 37 | 8 | 12 | 17 | 38 | 53 | -15 | 36 |
| 22 | Crawley TownCrawley | 38 | 8 | 9 | 21 | 40 | 71 | -31 | 33 |
| 23 | Cambridge UnitedCambridge | 38 | 7 | 9 | 22 | 36 | 62 | -26 | 30 |
| 24 | Shrewsbury TownShrewsbury | 37 | 7 | 7 | 23 | 34 | 62 | -28 | 28 |
| > League One Full Table | |||||||||
