The 26-year-old impressed for the Yellow Submarine on a season-long loan last season, and coach Marcelinho has made the move permanent.
The former Spain Under-21 defender was signed for a fee in the region of £2.5m, according to Spanish publication AS.
Ruiz represents the club's fifth signing of the summer, following Samuel Garcia, Leo Baptistao, Alphonse Areola and Samu Castillejo.
Restore Data
| Team | P | W | D | L | F | A | GD | PTS | |
| C | Real Madrid | 38 | 29 | 8 | 1 | 87 | 26 | 61 | 95 |
| 2 | Barcelona | 38 | 26 | 7 | 5 | 79 | 44 | 35 | 85 |
| 3 | GironaGirona | 38 | 25 | 6 | 7 | 85 | 46 | 39 | 81 |
| 4 | Atletico MadridAtletico | 38 | 24 | 4 | 10 | 70 | 43 | 27 | 76 |
| 5 | Athletic Bilbao | 38 | 19 | 11 | 8 | 61 | 37 | 24 | 68 |
| 6 | Real Sociedad | 38 | 16 | 12 | 10 | 51 | 39 | 12 | 60 |
| 7 | Real BetisBetis | 38 | 14 | 15 | 9 | 48 | 45 | 3 | 57 |
| 8 | Villarreal | 38 | 14 | 11 | 13 | 65 | 65 | 0 | 53 |
| 9 | Valencia | 38 | 13 | 10 | 15 | 40 | 45 | -5 | 49 |
| 10 | AlavesAlaves | 38 | 12 | 10 | 16 | 36 | 46 | -10 | 46 |
| 11 | Osasuna | 38 | 12 | 9 | 17 | 45 | 56 | -11 | 45 |
| 12 | Getafe | 38 | 10 | 13 | 15 | 42 | 54 | -12 | 43 |
| 13 | Celta Vigo | 38 | 10 | 11 | 17 | 46 | 57 | -11 | 41 |
| 14 | Sevilla | 38 | 10 | 11 | 17 | 48 | 54 | -6 | 41 |
| 15 | Mallorca | 38 | 8 | 16 | 14 | 33 | 44 | -11 | 40 |
| 16 | Las PalmasLas Palmas | 38 | 10 | 10 | 18 | 33 | 47 | -14 | 40 |
| 17 | Rayo Vallecano | 38 | 8 | 14 | 16 | 29 | 48 | -19 | 38 |
| R | CadizCadiz | 38 | 6 | 15 | 17 | 26 | 55 | -29 | 33 |
| R | Almeria | 38 | 3 | 12 | 23 | 43 | 75 | -32 | 21 |
| R | Granada | 38 | 4 | 9 | 25 | 38 | 79 | -41 | 21 |
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