Jo, who will be part of the squad which travels to Warsaw tomorrow, told the official club website: "I am delighted to join Celtic and I'm looking to make the very best of this great opportunity.
"I know what a magnificent club Celtic is and what the club means to the supporters - it is a real privilege to be here.
"I also know the manager very well and I am delighted to be teaming up with him again and, of course, the great squad of players at the club."
Celtic manager Ronny Deila added: "I'm really pleased that we have been able to add Jo to our squad. I know his qualities and I am sure he can add something new to the team.
"He is a player who will work hard and give everything to the team. I hope he can enjoy a successful time at Celtic."
The 23-year-old attacker joined Cardiff City in January from Molde, where he'd spent the previous three years. He's also spent a season on loan at Stromsgodset between 2010-11, during a three-year spell with Italian side Udinese.