The Westlife singer will front the interactive segment of the ITV1 daytime show throughout the summer.
Matt Johnson and Stephen Mulhern will stay as the main 'Hub' hosts while Only Way Is Essex star Gemma Collins and The X Factor's Johnny Robinson will also appear.
"I've always enjoyed This Morning, so I was delighted when I got the call to be part of the team," he told The Sun.
"The Hub is such a great way to interact with viewers discussing different topics - some serious and some light-hearted. I've also heard that it is a lovely team there, so I'm really looking forward to sharing the screen with Eamonn and Ruth."
This Morning executive producer Karl Newton said that Egan was "amazingly talented".
He added: "Our viewers will be in for a TV treat when he takes over 'The Hub'. He is sure to brighten up our overcast summer."
Kian Egan has also been filling in for Olly Murs by co-hosting The Xtra Factor with Caroline Flack during the Boot Camp stage of the show.
Westlife played their final concert together in Dublin last month.