29 year old Ridgewell was released by West Bromwich Albion this summer, and will head to the United States in the next month or so.
Ridgewell spent five years at Birmingham between 2007-2012, making 152 appearances in that period, in a mixed spell at the club, winning promotion, an amazing League Cup win over Arsenal in the February of 2011, before relegation to the Championship later that May.
And the signing was seen as a coup by Portland manager Caleb Porter.
Timbers coach Caleb Porter added: “Moving into the summer transfer window it was no secret we were looking to strengthen our squad with a top-level defender, and we are extremely pleased to announce the signing of Liam Ridgewell.
“Liam is still in his prime at only 29 years old and he’s been playing game in and out in the English Premier League versus many of the top attacking players in the world. This pedigree, along with his leadership qualities and technical ability as a left-sided central defender, makes him an ideal choice to enhance our backline for the second half of the season and beyond.”
Portland have also been linked with former Bolton Wanderers defender Zat Knight.