Prince Harry and Meghan said they hoped Catherine and her family could heal “privately and in peace”, in a brief statement.
On Friday, Catherine said that the news was a “huge shock” after an “incredibly tough couple of months”.
She said she was “well” and “getting stronger every day”.
Although details of the cancer have not been revealed, Kensington Palace says the princess is sure that she will make a full recovery.
Catherine went through an abdominal surgery in January, but it was not known that it would degrade to form a cancer.
Harry and Meghan stepped down as senior royals in January 2020.
They moved to California that June, saying they wanted space to raise their son, Archie. A second child, Lilibet, was born the following year.
King Charles has also recently been diagnosed with cancer, and Harry flew to London in early February to see his father a day after he began treatment.
In an interview, Harry said he was “grateful” for the brief time he got to spend with his father in London.
He also agreed families could be brought closer by such health concerns.
On Friday, US President Joe Biden’s press secretary, Karine Jean-Pierre wished the British royal a complete recovery soon.