Ridgewell will join the Timbers on a multi-year contract pending the receipt of his International Transfer Certificate (ITC) and P-1 visa.
The 29-year-old has spent 11 seasons in the English Premier League with Aston Villa, Birmingham City, and West Bromwich, and he is coming off a season in which he made 33 appearances for the Baggies in the EPL.
"Liam is a proven defender at the highest level and brings strong leadership qualities along with a wealth of experience," said Timbers general manager Gavin Wilkinson. "After attending a recent match at Providence Park, Liam was incredibly taken by the fans and atmosphere generated here in Portland and, despite receiving several attractive offers to stay in the Premiership, chose the Timbers due to his belief in the vision and football philosophy of the organization. He is keenly committed to helping this club exceed its already lofty goals this season, and for seasons to come."
Ridgewell's signing comes just two days after Portland exercised its purchase option on forward Fanendo Adi, making him the team's second Designated Player, while Ridgewell is now the third, joining Adi and midfielder Diego Valeri.