The Portuguese club have continued the process of revamping their squad in the summer by securing the services of the veteran Croatian midfielder
The Croatia international has signed a three-year contract which will keep him at the Estadio de Alvalade until 2015, with a €15 million (£11.8m) buy-out clause written into the deal.
Pranjic joins the Portuguese capital club on a free transfer following the expiry of his contract with Bayern Munich, whom he signed for in the summer of 2009.
The 30-year-old midfielder, who will be officially unveiled by Sporting on at 17:15 BST, featured for Croatia in their Euro 2012 group matches against Italy and Spain.
This is Sporting's fourth signing of the summer, as the club previously secured the services of Gelson Fernandes, Sunil Chhetri and Zakaria Labyad.