Skacel had been training with Hearts amid persistent speculation he would sign for a third spell, but the Tynecastle club saw their Scottish Premier League transfer embargo extended until December 23 on Wednesday following late payment of wages.
United quickly stepped in and signed the 33-year-old Czech international, who has scored in two Scottish Cup final wins for Hearts, until January 30.
Manager Peter Houston told the club's official website: "I feel Rudi can add an attacking spark to our team. Players of his quality do not present themselves to you too often and I am delighted to be able to bring him to Tannadice.
"The length of the deal gives both of us a chance to have a good look at each other and I feel our younger, less experienced players will benefit from Rudi's involvement at the club. His goalscoring ability should make him a fans' favourite."