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.
Would you like to place a soccer bet? Click here to get started in 3 easy steps.