There is an option for the Glasgow side to buy after the loan spell.
Berget has played just 11 minutes of Premier League football since joining Cardiff from Molde in January. He did manage 61 minutes of an FA Cup defeat against Wigan and that's all he's played for the first team.
The lad has never really had much of a chance and may turn out to be a good player. He's 23 years old and has played for his country.
Berget travels with Celtic to face Leiga Warsaw on Wednesday in the Champions League third qualifying round.
The Norwegian international told the official Celtic 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."