The Uruguayan, 30, has penned a three-year contract with the Premier League newcomers.
Britos made over 100 appearances for Napoli after joining the club from Bologna in 2011.
The centre-back is Watford's ninth new signing of the summer.
Quique Flores's side get their Premier League campaign underway with a trip to Everton on August 8.
Restore Data
| Team | P | W | D | L | F | A | GD | PTS | |
| 1 | Sheffield UnitedSheff Utd | 21 | 14 | 5 | 2 | 30 | 11 | 19 | 45 |
| 2 | Leeds UnitedLeeds | 21 | 12 | 6 | 3 | 37 | 15 | 22 | 42 |
| 3 | Burnley | 21 | 11 | 8 | 2 | 26 | 8 | 18 | 41 |
| 4 | Sunderland | 21 | 11 | 7 | 3 | 32 | 17 | 15 | 40 |
| 5 | Blackburn RoversBlackburn | 20 | 11 | 4 | 5 | 25 | 17 | 8 | 37 |
| 6 | Middlesbrough | 21 | 10 | 4 | 7 | 35 | 25 | 10 | 34 |
| 7 | Watford | 20 | 10 | 4 | 6 | 29 | 26 | 3 | 34 |
| 8 | West Bromwich AlbionWest Brom | 21 | 7 | 11 | 3 | 24 | 16 | 8 | 32 |
| 9 | Sheffield WednesdaySheff Weds | 21 | 8 | 5 | 8 | 26 | 30 | -4 | 29 |
| 10 | Swansea CitySwansea | 21 | 7 | 6 | 8 | 23 | 22 | 1 | 27 |
| 11 | Bristol City | 21 | 6 | 9 | 6 | 26 | 26 | 0 | 27 |
| 12 | Norwich CityNorwich | 21 | 6 | 8 | 7 | 36 | 32 | 4 | 26 |
| 13 | Millwall | 20 | 6 | 7 | 7 | 20 | 18 | 2 | 25 |
| 14 | Luton TownLuton | 22 | 7 | 4 | 11 | 25 | 38 | -13 | 25 |
| 15 | Derby CountyDerby | 22 | 6 | 6 | 10 | 27 | 28 | -1 | 24 |
| 16 | Coventry CityCoventry | 21 | 6 | 6 | 9 | 27 | 30 | -3 | 24 |
| 17 | Preston North EndPreston | 21 | 4 | 11 | 6 | 21 | 27 | -6 | 23 |
| 18 | Stoke CityStoke | 21 | 5 | 7 | 9 | 23 | 28 | -5 | 22 |
| 19 | Queens Park RangersQPR | 21 | 4 | 10 | 7 | 21 | 27 | -6 | 22 |
| 20 | Oxford UnitedOxford Utd | 20 | 4 | 6 | 10 | 21 | 33 | -12 | 18 |
| 21 | Cardiff CityCardiff | 20 | 4 | 6 | 10 | 19 | 32 | -13 | 18 |
| 22 | Portsmouth | 19 | 3 | 8 | 8 | 21 | 34 | -13 | 17 |
| 23 | Plymouth ArgylePlymouth | 20 | 4 | 5 | 11 | 19 | 42 | -23 | 17 |
| 24 | Hull City | 21 | 3 | 7 | 11 | 19 | 30 | -11 | 16 |
| > Championship Full Table | |||||||||
