The 21-year-old winger has joined the MLS Cup holders until May with a view to a permanent move.
The 21-year-old winger, who can also play at left-back or up front, is initially expected to provide the reigning MLS Cup champions with depth behind starting wide man Lucas Melano and Dairon Asprilla and has the option to sign for them on a permanent basis if he impresses.
Speaking to Portland’s official website, general manager Gavin Wilkinson said: “Acquiring Jack on loan with an option to retain long-term presents a great opportunity for both the player and the club.
“For his age he has very good experience at many levels, and we look forward to him joining the club.”
Timbers head coach Caleb Porter added: “Jack is a young player with good upside that brings versatility and depth to our roster. He is a left-sided player that can play either wing or left back.”
Barmby, the son of former England international Nick, began his youth career at Hull City before opting to join Manchester United in 2008 on the advice of his father.
The versatile midfielder, who has represented England himself at the U-16, U-18, U-19 and U-20 levels, was a highly-rated prospect while rising through United's youth ranks but ultimately left Old Trafford in the summer of 2014 to join then-Premier League newcomers Leicester on a free transfer.
However, he has so far failed to make a first-team appearance for the top-flight leaders, taking in loan spells at Championship strugglers Rotherham United and League Two's Notts County in the meantime.
His Foxes contract is set to expire in the summer, but if he makes a good impression at Providence Park over the coming months, his long-term future could well lie in MLS.