The Costa Rica international, who played 32 times for the Whites last season, has signed a contract that includes a £43m buyout clause.
Ruiz joined the Cottagers from Dutch outfit FC Twente in 2011 for £10.6m, scoring 13 times in 106 appearances during his time in England.
The 29-year-old almost joined Levante on loan back in February but the move fell through.
Restore Data
| Team | P | W | D | L | F | A | GD | PTS | |
| 1 | Sporting Lisbon | 11 | 11 | 0 | 0 | 39 | 5 | 34 | 33 |
| 2 | Porto | 11 | 9 | 0 | 2 | 28 | 8 | 20 | 27 |
| 3 | Benfica | 10 | 8 | 1 | 1 | 28 | 7 | 21 | 25 |
| 4 | Santa ClaraSanta Clara | 11 | 7 | 0 | 4 | 14 | 12 | 2 | 21 |
| 5 | Braga | 11 | 6 | 2 | 3 | 19 | 11 | 8 | 20 |
| 6 | Vitoria de GuimaraesGuimaraes | 11 | 5 | 3 | 3 | 12 | 11 | 1 | 18 |
| 7 | FamalicaoFamalicao | 11 | 4 | 5 | 2 | 12 | 9 | 3 | 17 |
| 8 | MoreirenseMoreirense | 11 | 5 | 2 | 4 | 15 | 14 | 1 | 17 |
| 9 | Casa Pia | 11 | 3 | 4 | 4 | 10 | 14 | -4 | 13 |
| 10 | Rio AveRio Ave | 11 | 3 | 3 | 5 | 10 | 20 | -10 | 12 |
| 11 | Gil Vicente | 11 | 2 | 4 | 5 | 14 | 19 | -5 | 10 |
| 12 | Estoril PraiaEstoril | 11 | 2 | 4 | 5 | 8 | 16 | -8 | 10 |
| 13 | AVS | 11 | 2 | 4 | 5 | 9 | 19 | -10 | 10 |
| 14 | BoavistaBoavista | 11 | 2 | 3 | 6 | 7 | 15 | -8 | 9 |
| 15 | Estrela Amadora | 11 | 2 | 3 | 6 | 10 | 20 | -10 | 9 |
| 16 | Nacional | 10 | 2 | 2 | 6 | 6 | 15 | -9 | 8 |
| 17 | AroucaArouca | 11 | 2 | 2 | 7 | 6 | 19 | -13 | 8 |
| 18 | SC FarenseFarense | 11 | 1 | 2 | 8 | 5 | 18 | -13 | 5 |
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