Adu, given a professional contract at 14-years-old and hailed as the 'American Pele' in some quarters, joined Benfica four years ago but never established himself at the Portuguese club.
The forward, who can also play as an attacking midfielder, underwent a series of loan spells, including one with French club Monaco, and spent last year in the Turkish second division.
Ghana-born Adu, who is still only 22, missed out on a place in the U.S. World Cup squad in 2010 but was recalled for this year's CONCACAF Gold Cup.
The deal with Philadelphia reunites Adu with Peter Nowak who was his coach at his first club D.C. United.
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