Celtic boss Ronny Deila says keeping his best players will take priority over acquisitions in the transfer window.
Chelsea striker Fernando Torres is understood to be set for a return to Atletico Madrid, according to Sky sources.
This season Tello has made 16 appearances in all competitions for Barca, having represented Spain at this summer's Olympics.
A Barcelona statement read: "Barcelona announce they have reached an agreement with the player Cristian Tello to extend his contract until June 30, 2016. His buy-out clause has been set at 10 million euros.
"The player will sign his contract on Monday, December 17, in the Barcelona offices."