Dupree, 36, has averaged 14.4 points, 6.6 rebounds, and 1.9 assists per game since entering the WNBA as the No. 6 pick in 2006. She ranks fifth all time in points (6,728) and sixth in total rebounds (3,071) in league history.
Dupree most recently played for the Indiana Fever following successful stints with the Phoenix Mercury and Chicago Sky.
"I can't tell you how excited I am to have to have an opportunity to play for the Seattle Storm," she said in a statement. "To be 15 years in and have an opportunity to play for a contender and compete for a back-to-back championship is awesome."
The reigning champs' frontcourt rotation will look slightly different from its 2020 version after forward Alysha Clark signed with the Washington Mystics and Crystal Langhorne recently announced her retirement.
However, the Storm frontcourt still boasts 2018 league MVP and two-time Finals MVP Breanna Stewart and Natasha Howard, the 2019 Defensive Player of the Year.