The Liga side also have an option to purchase the Croatia international, 31, on a permanent deal at the end of the season.
Pranjic will be presented to the press at Balaidos stadium at 5.30pm on Tuesday.
The move to Spain comes after Pranjic failed to impress following a summer move from Bayern Munich to Sporting.
The winger, who has won 45 caps for Croatia, has started only six league games for Sporting this season.
- Sports & Recreation