It was unclear as to when the discovery and removal took place.
About one-fifth of ovarian tumours contain foreign tissue, including hair, teeth, cartilage, fat and muscle. These tumours are named teratomas after the Greek word 'teras', meaning monster.
It is unknown what causes ovarian teratomas. One theory is that they arise when immature egg cells turn rogue, producing different body parts.
Mature teratomas, such as the one found in the young girl, are the most common type of ovarian germ cell tumour.
They are most often diagnosed in women during their reproductive years.
Mature teratomas are often called dermoid cysts. They are removed with surgery and the condition is then cured.
Masayuki Shintaku, who studied the tumour, from the Shiga Medical Centre for Adults, explained that brain cells are often found in ovarian teratomas.
But he added that it is extremely unusual for them to organise themselves into proper brain-like structures.
Angelique Riepsamen, at the University of New South Wales in Australia, agreed, telling the New Scientist: "Neural elements similar to that of the central nervous system are frequently reported in ovarian teratomas, but structures resembling the adult brain are rare."
The miniature brain even developed in such a way that electric impulses could transmit between neurons, just like in a normal brain, said Shintaku.
While the girl didn't experience any symptoms, there have been some reports of women with ovarian teratomas who have developed personality changes, paranoid thoughts, confusion, agitation, seizures or memory loss.
Some of these neurological symptoms can arise when the immune system recognises brain cells in the ovary as foreign and launches an attack because cells in the woman's real brain can end up being attacked as well, leading to inflammation.
In April 2015, a tumour was found growing on the brain of Yamini Karanam, an Indiana University PhD student.
The doctor was shocked during surgery to discover that the tumour was an embryonic twin, complete with bone, hair, and teeth, which Karanam later dubbed her 'evil twin.'
Dr Sean Grady, chairman of the department of neurosurgery at the Perelman School of Medicine at the University of Pennsylvania explained at the time that teratomas may not be remnants of twins.
He said: "The tissue elements found in an embryo are there, like hair or glandular structures, but it's not like there was a development of an initial human."
