The Aston Villa player will spend the season with the Scottish champions, who announced on Monday the deal includes an option to buy the 24-year-old next summer.
Midfielder Petrov spent seven years with Celtic before taking the opposite path to Tonev, by moving south of the border to join Villa.
As Tonev prepares for his big chance in the green and white hoops, the winger said: "Joining Celtic is unbelievable, and I feel really good about it
I'm delighted to have joined the club and I can't wait to meet up with my new team-mates and start playing for Celtic."
He added on Celtic's website: "I already knew all about Celtic
Everybody in Bulgaria knows Celtic
"I spoke to Stiliyan Petrov yesterday and he was very happy for me
He wished me good luck and I hope I can do as well for Celtic as Stiliyan did."
Source : PA
Source: PA