Timbers technical director/general manager Gavin Wilkinson made the announcement.
Zizzo, 23, marks his second season in Major League Soccer (MLS) after contributing one assist in 10 games (1 start) with Chivas USA in 2010. He was acquired by Chivas USA in a weighted lottery after signing with MLS in July 2010.
"Zizzo is a player we've been interested in pursuing for some time now," said Wilkinson. "We are delighted to add such a quality player to our roster."
A native of San Diego, Calif., Zizzo began his career in 2007 with Hannover 96 in the German Bundesliga. He made eight first-team appearances with the top-flight German club and also played for Hannover 96 II, the club's reserve team.
The midfielder played for the U.S. U-18 National Team and represented the United States at the 2007 FIFA U-20 World Cup, helping the team reach the quarterfinals. He earned his first cap for the U.S. Men's National Team during a friendly against Sweden on Aug. 22, 2007.
Zizzo played two years at UCLA (2005-06), earning NSCAA/adidas All-America honors after his freshman campaign. In his sophomore season, he was named Second Team All-American, scoring seven goals while leading the team with nine assists. After scoring a total of eight goals to go along with 14 assists in 43 games (39 starts) at UCLA, he left school following his sophomore season to begin his professional career in Germany.