The Catalan giants also loaned centre-back Clement Lenglet (29) to title rivals Atletico Madrid.
Brazilian forward Roque arrived from Athletico Paranaense in January after Barça signed him for around 30 million euros in July 2023.
He scored two goals in 16 games for the club as Barcelona watched rivals Real Madrid lift La Liga and the Champions League.
"Agreement reached between FC Barcelona and Real Betis for the loan of Vitor Roque until June 30 2025, with the option of another year," said Barcelona in a statement.
The Catalan giants confirmed Betis also have an option to buy the 19-year-old.
"You cannot say I don't count on him, that's totally wrong," Barca coach Hansi Flick told reporters Monday.
"It's a good chance for him to play, he's doing a lot of things really great... fingers crossed he's playing really well and improves."
French defender Lenglet joins Atletico until the end of the season, after previous loans to Tottenham and Aston Villa.
Barcelona have not been able to register summer signing Dani Olmo with La Liga yet, owing to their economic difficulties and the league's strict financial rules.
They let Ilkay Gundogan return to Manchester City last week and coach Hansi Flick was hopeful it would give the club enough room to register Olmo, but the playmaker was not available to face Athletic Bilbao on Saturday.
"This is a thing neither the player nor the coach can control," said Flick, ahead of Barca's league clash with Rayo Vallecano on Tuesday.
"We both know. We spoke about the situation. We know it's a little bit difficult here in Barcelona
"I believe in the club and... hopefully (he can be involved in) the match tomorrow. Of course, every player who cannot play for this reason is not happy."