The Portuguese forward had been without a club after terminating his contract with Porto by mutual consent earlier in the summer.
The 23-year-old flew to London on Wednesday to pass a medical with Fulham and put pen-to-paper on a three-year contract which will keep him at Craven Cottage until 2014.
"I am happy to have completed my transfer to Fulham and the Premier League today. This is an enormous opportunity for me and I’m looking forward to joining the team and entering the next phase of my development," the player told Fulham's website.
"It’s a wonderful challenge for me and I am keen to bring my qualities to Fulham and the Premier League."Sa, who has one senior cap to his name, joined Porto from Braga in the summer of 2009, but failed to shine at the Estadio do Dragao, having spent the last season on loan at CD Nacional.