The Angolan has put pen to paper on a four-year deal with the Sky Bet Championship club.
He arrives at Hillsborough for an undisclosed fee and becomes the Owls’ ninth new signing of the summer.
The 21-year-old is reunited is South Yorkshire with former Nacional team-mate Marco Matias.
Joao, who has who has scored three goals in as many appearances for the Portugal Under-21 team, began his career with Beira Mar Atletico Clube Almada after moving to Portugal as a teenager.
In 2011 he signed for Nacional and moved out on loan to third tier side SC Mirandela to gain first team experience.
After impressing with Mirandela, he returned to Nacional for the 2013/14 campaign and his form attracted the attention of a raft of clubs around Europe.