The two clubs have reached an agreement over a six-month loan, with only the final details to be finalized.
Under the terms of the agreement, the club from Galicia will have the option to make the transfer permanent at the end of the season.
Pranjic joined Sporting Lisbon on a free transfer from Bayern Munich in the summer, but hasn’t quite managed to prove his worth at Estadio Jose Alvalade.
The Croatia international made 9 league appearances for the Lions this season, while also featuring in all six Europa League group games.
Sporting Lisbon chiefs, nonetheless, allowed the player to leave the club on loan, with his high wages one of the main reasons behind their decision.
Pranjic, 31, can successfully operate as a left-sided defender or left midfielder, and his experience is likely to prove a major asset at Balaidos.