The Britain's Got Talent judge will co-host the morning show on UTV with Phillip Schofield four days a week from September, with Northern Ireland's Eamonn Holmes and wife Ruth Langsford continuing to appear on Fridays.
Speaking about her new role, Amanda said: "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.
"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, added: "Amanda is a great talent with warmth and charisma in abundance. We are thrilled to have her join the This Morning family."
Belfast's Christine Bleakley will also present some of the shows missed by Holly while she is on leave.
The presenter is expecting her third child in October, and has said she wants to continue working right up until the baby's birth.
Speaking ahead of their final show before their summer break on Friday, Phillip and Holly discussed the fact that it could be Holly's last before her maternity leave.
"It could be," said the mum-to-be."(Daughter) Belle was five weeks early, (son) Harry was two weeks early...
"I'm planning on coming back for a bit - but there's also a huge possibility I could go into labour in September on screen."
"Don't you worry I've been here long enough to know what to do. I am fully trained," joked Phillip. "It'll make a fantastic feature!"
"You won't even see my dust as I whizz out of here, contractions or not!" replied Holly.