The 29-year-old has played the entirety of his career in England with Midlands trio Aston Villa, Birmingham City and West Brom.
Ridgewell, a former England under-21 international, will move to Providence Park on a free transfer pending the necessary paperwork to make the switch to the MLS club..
A versatile defender who can operate either at left-back or centre-half, he will occupy an international slot with Timbers and become the third Designated Player on their roster.
"Moving into the summer transfer window it was no secret we were looking to strengthen our squad with a top-level defender," head coach Caleb Porter told the club's official website.
"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."
Ridgewell won the League Cup with Birmingham in 2011.
