She will share the role with the current host Andrea McLean and replaces Jackie Brambles, who announced her decision to leave the show to spend more time with her family earlier this year.
Thornton will make her first appearance on the show in September when it returns from its summer break.
She said: "I am honoured, thrilled and delighted to be joining one of the best teams on TV. I can't wait to get started as the show heads into its 10th year."
Fiona Keenaghan, ITV studios controller, said: "A great big warm welcome to Kate. I'm thrilled to bits that she is joining the Loose line up and I know she is going to fit in brilliantly."