He swooped for Scotland under 20 keeper Scott Fox to dig them out of a defensive crisis. The Celtic youth star has signed a loan deal until the end of the season.
United stalwart, Mark McGeown was ruled out with a back injury and with only teenager Fraser Stewart as cover, United boss Brian Reid was forced to send out an urgent request for help.
Fox, 20, spent the first half of the season on loan at Division 3 outfit East Fife, and went straight into the Ayr starting line up for Saturday's visit of Alloa.
He played for Scotland U20s in last summer's World Cup in Canada and he turned out for Ayr in a practice match against Partick Thistle at Somerset on Monday evening.
Reid, who is now searching for a right back, said: "Scott comes with a good pedigree and I am very pleased to have got him. He played for 45 minutes against Partick and looked very competent although he didn't have too much to do."
His first match saw United win 3-1 agaibst Alloa.