The Argentina midfielder completes a permanent move after a successful loan spell at Los Verdiblancos
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 (£21.6m/$28.2m) fee after being “convinced” by his performances.
“I am happy and enjoying the present that has given me many positive things,” Lo Celso told the club’s official website.
“[Betis] are in a situation that isn’t ideal at the moment, but we will fight until the end.
“We know what the club represent, what they aspire to and the goals that we set ourselves at the beginning of the season.
“The adaptation [at Betis] was very simple thanks to the leaders, coaching staff and teammates.
“When you feel comfortable, you transmit it onto the field and I always tried my best to help the team.”
Lo Celso began his career with boyhood club Rosario Central in his native Argentina before making the move to Paris in the summer of 2016.
He went on to make 38 appearances for PSG in Ligue 1 before his loan move to Betis last summer.
The Argentina international, who has 12 caps for La Albiceleste, is Betis’ top scorer with 15 goals in all competitions and was on target in the 3-2 defeat to city rivals Sevilla on Saturday.
“Luckily this year I scored a lot, but the most important thing is trying to help the team,” he added.
“On Sunday we have a very tough game at home against a rival that we already know.
“We have to win all the matches to stay up [in the European places] and it will not be easy, but the team is fine and we will fight until the end.”
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Betis are currently ninth in La Liga with six games to play as they battle to secure a place in Europe next season.
Quique Setien’s side take on Valencia at Benito Villamarin on Sunday. Victory would narrow the gap on sixth-placed Los Ches to just three points.