The presenter, who will replace Jackie Brambles after she decided to quit the programme in May, will host the daytime show alongside Andrea McLean.
Thornton 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 tenth year."
ITV Studios Controller of Daytime and Lifestyle Fiona Keenaghan added: "A great big warm welcome to Kate. I'm thrilled to bits that she is joining the Loose lineup and I know she is going to fit in brilliantly."
Loose Women will celebrate its tenth anniversary later this year.