The right-back has been pushing to leave Arsenal and secured a loan switch to La Liga late on deadline day.
The deal does not include an option to buy Bellerin, who has spent his entire career at Arsenal with the exception of a short loan spell at Watford in the 2013-14 season.
Bellerin has spoken in the past about his dad being a Real Betis fan and how he used to wear a Betis shirt when he played football growing up.
The 26-year-old has made 239 appearances in all competitions for Arsenal.
He has completed his medical in London on deadline day before flying to Spain.
Bellerin, who will spend the entire 2021-22 season on loan in LA Liga, was told by Arsenal manager Mikel Arteta that he was free to leave this summer after falling out of favour at Emirate Stadium.