The Poland international, who had more than a year to run on his previous deal, is now committed to the SPL club until the summer of 2015.
"I enjoy playing for Dundee United very much," said Cierzniak, who arrived from Cracovia in July. "Having settled in the area, it is an easy decision for me to sign on for a further year.
"I came here to improve as a player and I feel I am doing so. The cub, players and supporters have all helped with this and I look forward to exciting times over the next few years at Dundee United."
Manager Jackie McNamara said: "This extension is merited because Rado has shown that he is an excellent SPL goalkeeper. It can sometimes be difficult for a foreign player to come into a club and settle but he has done so superbly.
"He is a very good shot stopper and in my time here has shown some excellent aerial handling skills.
"It is a bonus for him to know his future is secure until at least May 2015 and it is great for us to know he wants to remain a United player until then also."