The two sides have been in talks over a deal for some time as Barca wanted to include an obligation for Betis to buy at the end of the season.
The Seville side would rather have the option to send him back, however, and are set to get their way after a meeting on Thursday.
Before he goes, Rafinha will likely extend his contract at Barca to 2021 as the Catalan side look to avoid losing him for a cut price.
The 25-year-old had been linked to a move to Inter, where he spent the second half of last season on loan after overcoming a serious knee injury, but the Italian side had to back out of the deal.
Rafinha joined Barcelona in 2006 and rose through the youth system to make his debut in 2011, going on loan to Celta two years later.
The Brazil international made his 80th appearance for the Camp Nou outfit when he started in the 2-1 win over Sevilla in the Supercopa de Espana, but did not feature in their opening two La Liga games of the season.
Should any deal fall through, ABC Sevilla report that the club have also reached an agreement with PSG over the loan signing of Giovani Lo Celso, though Betis are not expected to sign both players.