The 29-year-old has been on loan with the Turkish club from Metalist Kharkiv since the summer.
Under the terms of the loan agreement, the Black Eagles retained the right to sign the midfielder on a permanent basis - a clause that they have now decided to exercise.
It is believed that Sosa, who has made five appearances this season, has cost Besiktas around £1.5m.
Restore Data
| Team | P | W | D | L | F | A | GD | PTS | |
| 1 | GalatasarayGalatasaray | 36 | 32 | 3 | 1 | 89 | 24 | 65 | 99 |
| 2 | Fenerbahce | 36 | 29 | 6 | 1 | 92 | 31 | 61 | 93 |
| 3 | TrabzonsporTrabzonspor | 36 | 19 | 4 | 13 | 64 | 48 | 16 | 61 |
| 4 | Istanbul BasaksehirIstanbul | 36 | 17 | 7 | 12 | 51 | 40 | 11 | 58 |
| 5 | BesiktasBesiktas | 36 | 16 | 7 | 13 | 49 | 43 | 6 | 55 |
| 6 | KasimpasaKasimpasa | 36 | 15 | 7 | 14 | 60 | 64 | -4 | 52 |
| 7 | AlanyasporAlanyaspor | 36 | 12 | 14 | 10 | 51 | 48 | 3 | 50 |
| 8 | Caykur RizesporCaykur Rizespor | 36 | 14 | 7 | 15 | 48 | 56 | -8 | 49 |
| 9 | SivassporSivasspor | 36 | 12 | 12 | 12 | 42 | 52 | -10 | 48 |
| 10 | AntalyasporAntalyaspor | 36 | 11 | 12 | 13 | 41 | 47 | -6 | 45 |
| 11 | Adana Demirspor | 36 | 10 | 14 | 12 | 51 | 53 | -2 | 44 |
| 12 | Samsunspor | 36 | 11 | 9 | 16 | 40 | 48 | -8 | 42 |
| 13 | KayserisporKayserispor | 36 | 11 | 11 | 14 | 41 | 53 | -12 | 41 |
| 14 | KonyasporKonyaspor | 36 | 9 | 13 | 14 | 37 | 48 | -11 | 40 |
| 15 | AnkaragucuAnkaragucu | 36 | 8 | 15 | 13 | 44 | 48 | -4 | 39 |
| 16 | GaziantepGaziantep | 36 | 10 | 8 | 18 | 46 | 56 | -10 | 38 |
| 17 | Fatih KaragumrukKaragumruk | 36 | 9 | 10 | 17 | 45 | 48 | -3 | 37 |
| 18 | Hatayspor | 36 | 8 | 13 | 15 | 41 | 50 | -9 | 37 |
| 19 | Pendikspor | 36 | 9 | 9 | 18 | 42 | 72 | -30 | 36 |
| R | Istanbulspor | 36 | 4 | 7 | 25 | 26 | 71 | -45 | 16 |
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