"In January, I underwent major abdominal surgery in London and at the time, it was thought that my condition was non-cancerous," Kate said in a video message.
She added: "The surgery was successful. However, tests after the operation found cancer had been present. My medical team therefore advised that I should undergo a course of preventative chemotherapy and I am now in the early stages of that treatment."
Kate also said in her message that she was "well and getting stronger every day by focusing on the things that will help me heal; in my mind, body and spirits."
She asked for "space and privacy" while she completed her treatment.
"At this time, I am also thinking of all those whose lives have been affected by cancer," Kate said. "For everyone facing this disease,
in whatever form, please do not lose faith or hope. You are not alone."
It was not announced what type of cancer it was, or at what stage it was caught, news outlets reported.
British Prime Minister Rishi Sunak said in a statement that "the Princess of Wales has the love and support of the whole country as she continues her recovery."
Buckingham Palace said last month that Britain's King Charles had been diagnosed with cancer and would postpone public-facing duties while undergoing treatment.
(gs)
Source: PAP, Reuters, BBC