Cottagers manager Rene Meulensteen said Friday the 30-year-old forward has had a medical test.
Dempsey scored 60 goals in 225 games for Fulham from 2007-12, including 50 goals in 184 Premier League matches. He spent a season at Tottenham before moving to Major League Soccer in August.
Dempsey could spend about two months at Fulham ahead of Seattle's 2014 season opener against defending MLS champion Sporting Kansas City.
"Clint will be a boost for us," Meulensteen said.
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