Startled researchers believe the 22-year-old's case is the first of its kind.
Between one to five per cent of the population are believed to suffer from the condition, which is clinically referred to as supernumerary breast tissue.
If present, extra nipples tend to be found above the waist.
Pop star Lily Allen famously has an extra nipple on her left breast. Bond baddie Scaramanga also suffered from a similar condition.
Rare ... third nipples are usually found above the waist
But medical staff believe the nipple's appearance so far down the body, in this case, is a total one off.
It features in a report in scientific journal Dermatology Online.
Its authors wrote: "A 22-year-old woman sought medical care for a lesion in the plantar region of her left foot, a well-formed nipple surrounded by areola and hair.
Showing off ... Lily Allen shows off third nipple
"Microscopic examination of the dermis showed hair follicles, eccrine glands, and sebaceous glands.
"Fat tissue was noted at the base of the lesion. Clinical and histopathologic findings were consistent with the diagnosis of supernumerary breast tissue, also known as pseudomamma.
"To our knowledge, this is the first report of supernumerary breast tissue on the foot."
They went on to report it had been there since birth. There were no complaints of pain, and it remained looking and feeling the same. No one in her family had a similar condition. And it now measures four centimetres in diameter.
The report added: "Supernumerary breast tissue, known as SBT, is rarely found beyond the mammary line. But they have been seen before on the back, shoulder, face, and even thigh."