The right-back has urged to leave Arsenal and secured a loan move to La Liga late on the deadline.
The deal does not include an option to buy Bellerin, who has had his entire career with the exception of a brief loan at Watford spent at Arsenal in the 2013-14 season.
Bellerin has spoken in the past about his father being a Real Betis fan and how he used to wear a Betis shirt when he played football.
Bellerin, who will be on loan in LA Liga for the entire 2021-22 season, has been told by Arsenal manager Mikel Arteta that he will be free to go after disfavor at the Emirates Stadium this summer.
To confirm the move, Arsenal said: “Everyone at Arsenal wishes Hector the best of luck with Real Betis this season.”