The club said the 18-year-old Real Salt Lake player had to pass a medical before joining the Lisbon club.

Ghanaian-born forward Adu became the youngest player to represent the U.S. senior team when he made his debut against Canada in a friendly in January 2006 aged 16.

Champions League
Pirlo eyes final repeat
06/08/2007 AT 14:10

He has been linked with Europe's greatest clubs since bursting onto the scene aged just 13, but interest from the likes of Manchester United and Internazionale appears to have cooled.

Benfica finished third last season in the 16-team standings and will play the third qualifying round of the Champions League next month.

Champions League
Round-up: Salzburg look strong
01/08/2007 AT 21:22
Champions League
Round-up: Rangers take lead
31/07/2007 AT 21:12