The Britain's Got Talent judge will be joining Phillip Schofield on the famous sofa for four days a week while Holly goes off to have baby number three.
Amanda, who is mum to Lexi, eight, and Hollie, two, says she is 'over the moon' to be joining the popular daytime show.
"I'm over the moon to get the chance to present one of my favourite shows on TV and I'm thrilled to be keeping Holly's seat on the sofa warm when she goes off on maternity leave," she said in a statement.
"I have always been a huge fan of This Morning and love presenting with Phillip so I really can't wait to get started."
ITV Daytime managing executive producer Emma Gormley said: "Amanda is a great talent with warmth and charisma in abundance. We are thrilled to have her join the This Morning family."
Christine Bleakley – who has co-hosted the show with Phillip in the past - will also present some of the shows while Holly is away.
Holly's third baby is due in September, and she is already mum to Harry, five, and Belle, three. The mum-to-be always has time off from This Morning over the summer, and has said this year will be no different, despite her maternity leave starting soon after her break.
"I always have a big chunk of time off over the summer, which I guard with my life" Holly has said. "We'll always go away somewhere for a bit but it's mainly about being at home with the kids, doing nothing and being a mum - I love it!'"
Holly was presenting This Morning while she was pregnant with Belle, and Jenni Falconer covered her maternity leave.
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