Ferguson, 64, who was married to Prince Andrew, was treated for breast cancer last year and has now been diagnosed with malignant melanoma after several moles were removed.
"She is undergoing further investigations to ensure that this has been caught in the early stages," Sky News quoted her spokesman as saying.
"Clearly, another diagnosis so soon after treatment for breast cancer has been distressing, but the duchess remains in good spirits."
Ferguson, known as Fergie, has spoken of the importance of coming forward for cancer checks after her initial breast cancer surgery.
Her new diagnosis was reported as King Charles, 75, prepares for a "corrective procedure" for an enlarged prostate this week.
On Wednesday royal officials also announced that Catherine, wife of Charles' heir Prince William, had undergone successful planned abdominal surgery and will be in hospital for up to two weeks.
