To acquire the 20-year-old, Portland sent DCU $100,000 in general allocation money, $100,000 in targeted allocation money, a 2018 international roster spot and a second-round pick in the 2020 MLS SuperDraft.
Williamson was a DCU academy player but did not come to an agreement with the club on a homegrown deal after spending three seasons at the University of Maryland.
After exploring options in Europe, Williamson will instead begin his pro career with the Cascadia side.
“Eryk is a talented young American player that we believe possesses the skillset and tools to one day become an impact player in MLS," Timbers general manager Gavin Wilkinson said in a club release.
"This addition gives our club an exciting prospect for the future. We look forward to Eryk joining our group in preseason and getting to work with his new teammates and coaching staff.”
Williamson has appeared for the United States at both the U-18 and U-20 levels, including four starts at the 2017 U-20 World Cup last summer, where he helped the U.S. to a quarterfinal appearance.