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 | 37 | 29 | 7 | 1 | 87 | 26 | 61 | 94 |
| 2 | Barcelona | 37 | 25 | 7 | 5 | 77 | 43 | 34 | 82 |
| 3 | GironaGirona | 37 | 24 | 6 | 7 | 78 | 46 | 32 | 78 |
| 4 | Atletico MadridAtletico | 37 | 23 | 4 | 10 | 68 | 43 | 25 | 73 |
| 5 | Athletic Bilbao | 37 | 18 | 11 | 8 | 60 | 37 | 23 | 65 |
| 6 | Real Sociedad | 37 | 16 | 12 | 9 | 51 | 37 | 14 | 60 |
| 7 | Real BetisBetis | 37 | 14 | 14 | 9 | 48 | 45 | 3 | 56 |
| 8 | Villarreal | 37 | 14 | 10 | 13 | 64 | 64 | 0 | 52 |
| 9 | Valencia | 37 | 13 | 9 | 15 | 38 | 43 | -5 | 48 |
| 10 | AlavesAlaves | 37 | 12 | 9 | 16 | 35 | 45 | -10 | 45 |
| 11 | Osasuna | 37 | 12 | 8 | 17 | 44 | 55 | -11 | 44 |
| 12 | Getafe | 37 | 10 | 13 | 14 | 41 | 52 | -11 | 43 |
| 13 | Sevilla | 37 | 10 | 11 | 16 | 47 | 52 | -5 | 41 |
| 14 | Celta Vigo | 37 | 10 | 10 | 17 | 44 | 55 | -11 | 40 |
| 15 | Las PalmasLas Palmas | 37 | 10 | 9 | 18 | 32 | 46 | -14 | 39 |
| 16 | Rayo Vallecano | 37 | 8 | 14 | 15 | 29 | 47 | -18 | 38 |
| 17 | Mallorca | 37 | 7 | 16 | 14 | 31 | 43 | -12 | 37 |
| 18 | CadizCadiz | 37 | 6 | 15 | 16 | 25 | 49 | -24 | 33 |
| R | Granada | 37 | 4 | 9 | 24 | 38 | 72 | -34 | 21 |
| R | Almeria | 37 | 2 | 12 | 23 | 37 | 74 | -37 | 18 |
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