On July 28, 2007, Adu opted out of playing for Real in their regular-season match, and later that day, boarded a plane to Portugal to negotiate with Benfica. On July 30, Benfica issued an official statement announcing that Adu had been transferred to their club. The following day, the signing was completed and he trained with the team in Lisbon. Adu cost Benfica US$2 million. On August 14, 2007, Adu made his debut with Benfica against Copenhagen in a UEFA Champions League qualifying match, coming into the game in the 37th minute as a substitute.
Adu captained the United States U-20 men's national team in January 2007 as it qualified for the 2007 FIFA U-20 World Cup in Canada. By playing in the 2007 tournament finals, held from June to July, Adu became only the second player in the world to play in three FIFA U-20 World Cups. On July 3, 2007, Adu scored a hat-trick in U.S.'s 6–1 victory over Poland in the group stage of the tournament. This accomplishment made him the first player to ever score a hat trick in both the U-20 and U-17 World Cups. In the following 2–1 victory over Brazil Adu assisted on both USA goals by Jozy Altidore.
Adu made his first start for the senior international team in a friendly against South Africa on November 17, 2007.