By
The 23-year-old midfielder has scored 15 goals in all competitions in an impressive first season in Seville.
"Betis have activated the option that accompanied the loan of the midfielder," PSG said in a statement. "He is permanently transferred to the Andalusian club."
Lo Celso has joined Betis on a four-year contract for a reported fee of 22 million euros ($24.85 million), plus the three million for his original loan.
Betis sit ninth in the Spanish top flight, just three points off a Europa League qualifying place.
Lo Celso featured consistently for PSG last term, playing 48 matches before leaving in the close season.