The 21-year-old left-back, who can also operate on the right, joins for a total fee of 6 million euros - 4 million euros paid upfront and the remaining 2 million in instalments.
The deal includes a buy option that Como activated after a strong second half of the 2023-24 season.
Valle's path to the shores of Lake Como has been shaped by several developmental stints. He spent the 2022-23 season on loan at Levante, initially starting regularly before falling out of favour following a managerial change.
The following campaign began at Celtic, where he shared minutes with Greg Taylor. However, it was his January 2024 switch to Como that proved transformative.
Under the guidance of head coach Cesc Fabregas - a fellow La Masia product - Valle quickly became a mainstay in the starting XI.
His performances were instrumental in convincing the club to trigger the buy clause, despite interest from Real Betis and Borussia Dortmund.
Valle, who featured in Barcelona's 2023 preseason under Xavi Hernandez, appeared to be on the cusp of breaking through. But with the arrival of Hansi Flick and Gerard Martin overtaking him in the full-back hierarchy, his opportunities at the Camp Nou dwindled.
A move away was always likely, and sources close to the player suggest he was keen to remain at Como, where he has found both form and favour.