The 47-year-old star will host with show regular Rochelle Humes as the presenting duo take their summer break.
Dermot, who recently welcomed baby son Kasper with wife Dee Koppang, will stand in alongside Rochelle on the sofa on Fridays, while Alison will be his co-host for another.
Meanwhile, Rochelle, 31, will also present with former Strictly winner Ore Oduba.
The remaining dates will be covered by Ruth Langsford and Eamonn Holmes, who take to screens from Monday to Thursday.
Announcing the news on Twitter, This Morning wrote: "As @EamonnHolmes and @RuthieeL kick off your summer This Morning, we've announced a whole host of presenting partnerships who'll be joining them! @RochelleHumes @AlisonHammond @OreOduba @radioleary #ThisMorning."
This Morning presenters this summer
Fans rushed to comment, with one writing: "I enjoy the summer presenters so much more!"
Another agreed: "Yay for Ruth & Eamonn!! Here comes Summer."
While others weren't as convinced, saying: "Of which I'll be avoiding every single one of them until Holly and Phil return first week of September."
A second added: "Well that will stay switched off then!!! Haven’t watched it for months and this lot won’t be tempting me back!!!"
'A different summer'
Speaking on the change, show editor Martin Frizell said: "We’ve been inundated with viewers’ positive feedback throughout lockdown on how This Morning has provided a much-needed comfort blanket.”Eamonn - who usually presents This Morning on a Friday - added he hoped the show would bring a "sense of normality" to viewers.
He explained: “It’s set to be a very different summer for us all, so we hope that here on This Morning we can continue to provide some sense of normality, and lots of craziness of course.”
While wife Ruth said: “Like many viewers I’ve finally had my hair done – hurrah – and am ready to face the summer.”
Why aren't Holly and Phil on This Morning?Holly and Phillip are officially on their summer break after presenting the daytime show two metres apart in recent months.
As the pair finished up last week, they enjoyed wine and ice cream to celebrate their last show before the break.
Phil said: "We don't often run credits but we will this year because we want to thank the amazing crew.
He added: "Hopefully in September I can give you a bloody hug!"
Holly shared a heartfelt message on her Instagram thanking Phil as well as the whole cast and crew behind ITV's This Morning.
The star wrote: "Thank you .... thank you for staying with us over the last 109 days...
"When we began this new way of broadcasting, we had no idea how long we would be able to come in, or whether it would be our last time broadcasting from the studio during lockdown.
"The team have adapted and had plan a, b, c and d in place just in case...
"Some days we didn’t know if we’d have the content to fill the show, but somehow we always managed it and even had a few laughs along the way."
She later added a special thank you to co-host Phil: "But mostly to @schofe for metaphorically holding my hand and reliably being socially distanced shoulder to shoulder with me...
"What a strange time... Feeling incredibly grateful... Huge love, thank you again... See you in September."
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