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 | |
| 1 | Real Madrid | 32 | 25 | 6 | 1 | 70 | 22 | 48 | 81 |
| 2 | Barcelona | 32 | 21 | 7 | 4 | 64 | 37 | 27 | 70 |
| 3 | GironaGirona | 32 | 21 | 5 | 6 | 67 | 40 | 27 | 68 |
| 4 | Atletico MadridAtletico | 32 | 19 | 4 | 9 | 59 | 38 | 21 | 61 |
| 5 | Athletic Bilbao | 32 | 16 | 10 | 6 | 52 | 30 | 22 | 58 |
| 6 | Real Sociedad | 32 | 13 | 12 | 7 | 46 | 34 | 12 | 51 |
| 7 | Real BetisBetis | 32 | 12 | 12 | 8 | 40 | 38 | 2 | 48 |
| 8 | Valencia | 32 | 13 | 8 | 11 | 35 | 34 | 1 | 47 |
| 9 | Villarreal | 32 | 11 | 9 | 12 | 51 | 55 | -4 | 42 |
| 10 | Getafe | 32 | 9 | 13 | 10 | 38 | 44 | -6 | 40 |
| 11 | Osasuna | 32 | 11 | 6 | 15 | 37 | 46 | -9 | 39 |
| 12 | Sevilla | 32 | 9 | 10 | 13 | 41 | 45 | -4 | 37 |
| 13 | Las PalmasLas Palmas | 32 | 10 | 7 | 15 | 30 | 39 | -9 | 37 |
| 14 | AlavesAlaves | 32 | 9 | 8 | 15 | 28 | 38 | -10 | 35 |
| 15 | Rayo Vallecano | 32 | 7 | 13 | 12 | 27 | 39 | -12 | 34 |
| 16 | Celta Vigo | 32 | 7 | 10 | 15 | 37 | 47 | -10 | 31 |
| 17 | Mallorca | 32 | 6 | 13 | 13 | 26 | 38 | -12 | 31 |
| 18 | CadizCadiz | 32 | 4 | 13 | 15 | 22 | 45 | -23 | 25 |
| 19 | Granada | 32 | 3 | 9 | 20 | 33 | 61 | -28 | 18 |
| 20 | Almeria | 32 | 1 | 11 | 20 | 31 | 64 | -33 | 14 |
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