Townsend has joined the Scottish Premiership club on a season-long loan.
The Hull academy graduate has spent time on loan at Grimsby, Chesterfield and Carlisle in recent seasons.
Dundee United manager Jackie McNamara told the club's official website: "I was delighted to secure Conor so quickly. It was important to get him in early so he can settle in before the start of the competitive action.
"He is a really good defender who will do a job here. His athleticism and height will enhance our squad."
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