The 31-year-old centre-back is set to join the Championship club on 2 January for a six-week spell.
The 31-year-old defender is set to join Brighton & Hove Albion on loan from 2 January until 13 February, with the Championship club confirming the move on their official website.
Brighton boss Chris Hughton previously managed the veteran centre-back at Birmingham City and believes that his experience of over 350 senior appearances will give the team a big boost over the crucial winter period.
"I know Liam very well from my time as manager at Birmingham City, and he is an excellent and important addition for us,” Hughton told seagulls.co.uk. "He will bring extra experience to our defence at a crucial time, and will give me an option both in the centre of defence and at left-back.
"Liam has played the vast majority of his career in the Premier League - with more than 350 senior appearances; he's also got experience of the Europa League and Championship from his time at Birmingham."
Really happy to have join Brighton looking forward to the games coming up. Thanks for all the good tweets pic.twitter.com/MdhIk9UwGF— Ridgy 6 (@liam_ridgewell) December 30, 2015
Ridgewell originally joined the Timbers in June 2014 as a Designated Player after leaving West Bromwich Albion on a free transfer and has since made 52 MLS appearances for the club, while recording three goals and two assists in the process.
The former Aston Villa man captained the Oregon outfit for most of 2015 with regular skipper Will Johnson struggling for fitness and his reliable performances in the heart of defence alongside centre-back partner Nat Borchers proved instrumental to their MLS Cup success.
His impending Brighton spell will not be the first offseason loan that the ex-England Under-21 international has taken in during his time in Portland, as he also spent last winter in the Championship at Wigan Athletic, making six appearances.