Having signed for PSG in 2016, Lo Celso made 48 appearances in all competitions last term but has found his game time limited since Thomas Tuchel's arrival at the club.
In order to gain regular first-team football the 22-year-old has secured a temporary switch away from Paris, joining Quique Setien's side on a season-long loan.
"I'm very happy to be at a club with such a great history," Lo Celso told Betis' official website.
"One of the things that made me come was the club's project. Any player would get motivated by it."
Lo Celso could make his Betis debut in the derby clash with Sevilla on Sunday.