PSG are on 69 points with four games left with Monaco trailing by 11 points
The 24-year-old Tello, who was loaned to Portuguese club Porto last season, made his Barca debut in 2011 and scored 20 goals in 86 appearances for the club.
"Thank you Fiorentina for the trust and for giving me the opportunity to play in the most beautiful city in Italy. I will not disappoint you," Spaniard Tello said on Twitter.
Third-placed Fiorentina play Genoa, who are 16th in Serie A, on Sunday.