The Spanish side signed midfielder Lo Celso on loan in August 2018 and the 23-year-old has now agreed a contract through to 2023.
Betis CEO Jose Miguel Lopez Catalan confirmed in November the club's intention to pay the required €25million fee after being "convinced" by his performances.
The Argentina international is Betis' top scorer with 15 goals in all competitions and was on target in the 3-2 derby defeat to Sevilla on Sunday (AEST).
He made 38 appearances for PSG in Ligue 1 after arriving from boyhood club Rosario Central in 2016.
Betis sits ninth in LaLiga with six games to play.