Willis, who is currently co-host of Big Brother's Bit on the Side, has been filling in on the ITV1 morning show on Fridays over the summer as part of a team of guest presenters.
She will host The Hub on Mondays and Tuesdays, with Stephen Mulhern and Matt Johnson taking over during the rest of the week.
"2012 has been crazy [and] co-hosting on This Morning has most definitely been a highlight for me," The Sun quotes Willis as saying.
"Working with Philip [Schofield], Holly [Willoughby], Eamonn [Holmes] and Ruth [Langsford] has been nothing but a pleasure, so knowing I will officially be part of the family from October is really exciting. [I] can't wait to get cracking."
This Morning editor Adam Vandermark described Willis as "a much-respected part of the This Morning family", saying that he was "thrilled" she had agreed to join the team and hailing her "natural warmth with the audience".
Coleen Nolan, who previously hosted the Hub segment, left This Morning in June after two years in order to pursue new projects.