In the video, the former Kate Middleton, 42, said the diagnosis was a "huge shock after an incredibly tough couple of months."
The cancer was found during testing done after she successfully underwent major abdominal surgery in January, Kate said. She did not specify the type of cancer or its stage.
Kate's health and well-being have been the subject of speculation since she virtually disappeared from public view in late December.
On Jan. 17, Kensington Palace announced that Kate was recovering from a planned abdominal surgery. The princess "is unlikely to return to public duties until after Easter," on March 31, the palace said at the time. Even so, her window of absence fueled online conspiracy theories about the princess.
It's another blow in a recent bout of bad news for the royal family. Last month, the palace revealed that King Charles III, 75, had been diagnosed with cancer.
Before Friday, the most recent update from Kate came earlier this month with the release of a photo of the Princess of Wales and her children — Princess Charlotte, Prince Louis and Prince George — and credited to husband Prince William, was released on Mother's Day in the U.K. The photo was later revealed to be doctored, with Kate taking the blame for an amateur editing job.
"The Princess of Wales has the love and support of the whole country as she continues her recovery," he said.
Shortly after the news broke, White House Press Secretary Karine Jean-Pierre began her media briefing with well wishes for Kate's full recovery.
"We are incredibly sad to hear of the news," she said. "We want to make sure that we certainly respect their privacy at this incredibly difficult time."