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."
Skacel, who has also played for clubs including Southampton, Marseille, Slavia Prague and Hertha Berlin, added: "Within 10 minutes of meeting the manager, I had decided I wanted to come here; it was an easy decision.
"I have kept fit training with a few of my previous clubs over the last few months so it is just match fitness I have to get up to speed with.
"For both the club and myself it is a good deal. I can play a few games and hopefully help the team climb the table.
"I am ready to be involved but that will be the decision of the manager.
"In January, we can then look at things and see if it suits us both to extend the contract."