The US international arrives from Tottenham Hotspur for the remainder of the season.
As part of the deal, Celtic have the option to make the deal for the centre-back permanent.
The 23-year-old has played five times for Spurs, most recently in European competition this season.
Celtic are the player’s seventh loan spell.
He said: “I am absolutely delighted to agree this move to Celtic, it is a really exciting opportunity for me and I am desperate to meet my new team-mates and get going for the season ahead.
“I know I am going to a huge Club which means so much to so many people and I will be doing all I can to bring our fans the success which their support deserves.”