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 | 36 | 29 | 6 | 1 | 83 | 22 | 61 | 93 |
| 2 | Barcelona | 36 | 24 | 7 | 5 | 74 | 43 | 31 | 79 |
| 3 | GironaGirona | 36 | 23 | 6 | 7 | 75 | 45 | 30 | 75 |
| 4 | Atletico MadridAtletico | 36 | 23 | 4 | 9 | 67 | 39 | 28 | 73 |
| 5 | Athletic Bilbao | 36 | 17 | 11 | 8 | 58 | 37 | 21 | 62 |
| 6 | Real Sociedad | 36 | 15 | 12 | 9 | 49 | 37 | 12 | 57 |
| 7 | Real BetisBetis | 36 | 14 | 14 | 8 | 48 | 43 | 5 | 56 |
| 8 | Villarreal | 36 | 14 | 9 | 13 | 60 | 60 | 0 | 51 |
| 9 | Valencia | 36 | 13 | 9 | 14 | 37 | 40 | -3 | 48 |
| 10 | Getafe | 36 | 10 | 13 | 13 | 41 | 51 | -10 | 43 |
| 11 | AlavesAlaves | 36 | 11 | 9 | 16 | 34 | 45 | -11 | 42 |
| 12 | Sevilla | 36 | 10 | 11 | 15 | 47 | 50 | -3 | 41 |
| 13 | Osasuna | 36 | 11 | 8 | 17 | 40 | 54 | -14 | 41 |
| 14 | Las PalmasLas Palmas | 36 | 10 | 8 | 18 | 32 | 46 | -14 | 38 |
| 15 | Rayo Vallecano | 36 | 8 | 14 | 14 | 29 | 44 | -15 | 38 |
| 16 | Celta Vigo | 36 | 9 | 10 | 17 | 42 | 54 | -12 | 37 |
| 17 | Mallorca | 36 | 7 | 15 | 14 | 29 | 41 | -12 | 36 |
| 18 | CadizCadiz | 36 | 6 | 14 | 16 | 25 | 49 | -24 | 32 |
| R | Granada | 36 | 4 | 9 | 23 | 37 | 70 | -33 | 21 |
| R | Almeria | 36 | 2 | 11 | 23 | 35 | 72 | -37 | 17 |
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