The princess made the announcement in a video which shows her enjoying time with her family - Prince William, Prince George, Princess Charlotte and Prince Louis.
In the video, she said: "As the summer comes to an end, I cannot tell you what a relief it is to have finally completed my chemotherapy treatment.
“The last nine months have been incredibly tough for us as a family. Life as you know it can change in an instant and we have had to find a way to navigate the stormy waters and road unknown."
The princess announced in March that pre-cancerous cells had been found following abdominal surgery.
"The cancer journey is complex, scary and unpredictable for everyone, especially those closest to you,” she explains in the video.
“This time has above all reminded William and me to reflect and be grateful for the simple yet important things in life, which so many of us often take for granted. Of simply loving and being loved.
"Doing what I can to stay cancer free is now my focus.”
Kate's parents, Carole and Michael Middleton, are also featured in the video.
