It's the forward's second stint at Tannadice after a loan spell from Aberdeen in 2016/17 and he joins fellow new boy Aidan Nesbitt ahead of Saturday's game with Dunfermline.
Manager Robbie Neilson told the club website: “Cammy will bring us real attacking intent. He can create and score goals and has the experience of winning the Championship previously.”
United have also snapped up their former midfielder Morgaro Gomis. The 33-year-old returns to Tannadice after spells with Hearts and Motherwell, which were followed by stints in Malaysia and Oman.
