The former Manchester United winger began his career at Sporting, leaving in 2007 only to return on loan from United in 2014.
The 31-year-old arrives from Valencia having been loaned to Lazio last season, Sporting announced in a statement.
Nani, his country’s third most-capped player, missed out on the World Cup where Portugal lost to Uruguay in the last 16.
Download our app and read this and other great stories on the move. Available for Android and iOS.