While Ferguson has appeared on This Morning before, she has never hosted and her trial period will also see her take on some behind the scenes responsibilities alongside lead producer Martin Frizell.
In a statement, Ferguson said: "I have appeared on This Morning before, but this time I am coming back in a very different role. I love live TV and am an enormous fan of the show, and the whole team is always lovely to deal with."
The former royal added: "I’m looking forward to being there for the whole morning, and thrilled to be guest editing the show which will mean getting to explore some topics that I’m passionate about."
Among the topics to be explored on the show – which will still feature Dermot O'Leary and Alison Hammond – will be an interview with Cliff Richard, the importance of defibrillators and how best to protect your skin.
Frizell also commented on the signing of Ferguson: "We are all delighted Sarah, Duchess of York wanted to come back here and I’m very happy for her to make some of my decisions on Monday. She has a great understanding of the show and how television works and I feel viewers will be really interested in the running order she is putting together."
This Morning has been trialling a number of presenters to permanently replace Phillip Schofield and Holly Willoughby following their departures this year.
With regular relief presenter Josie Gibson currently starring in I'm a Celebrity... Get Me Out of Here, Cat Deeley and Rylan Clark have both appeared on This Morning.
Ben Shephard is also in the frame even after plans were scrapped for him to replace Schofield earlier this year.
Schofield left the breakfast show in May after admitting to having an affair with a much younger colleague with Willoughby stepping down in October following much criticism of her position and also following news that she the target in an alleged kidnapping plot.
Watch below: Sarah Ferguson leads tributes to King Charles.