The 20-year-old, who came through the Spanish club’s La Masia academy, was an unused substitute in Barcelona’s first two La Liga matches.
He becomes the second fresh face to join Celtic this season alongside Kasper Schmeichel, while Adam Idah and Paulo Bernardo both completed moves after loan spells last term.
Celtic sold Matt O’Riley to Brighton this week and manager Brendan Rodgers has been clear more players would arrive before Friday’s deadline.
Valle played 29 times in the Spanish second tier last season while on loan at Levante and will provide competition for Greg Taylor.
“His footballing style will complement our attacking play as well as giving us more options in our backline,” said Rodgers.
“Alex has been brought up in a real high-quality footballing environment learning all the good habits and skills associated with any top team.
He is a player with some tremendous attributes.”
Valle says he is excited at the opportunity to play for an “historic club”.
“They have given me a lot of confidence to come here, so I will try to do my best to return the confidence,” the Spain youth international added.
“I’ve heard a lot about the fans and and what we can experience in this stadium, so I’m just excited to see it in real life.”
