Mr Ghosn is kept in an unheated cell, lost almost seven pounds and is given only rice and barley to eat, his wife claims.
In a nine-page letter, Carole Ghosn said: "My husband's is a case study in the realities of this draconian system".
Japanese authorities arrested Carlos Ghosn on 19 November and charged him with under-reporting his income by about 5 billion yen (£34m) over five years from 2010 to 2015.
Mrs Ghosn wrote to Japan's director of Human Rights Watch, calling on the organisation to "shine a light on the harsh treatment of my husband and the human rights-related inequities inflicted upon him by the Japanese justice system".
Mr Ghosn is being held in an unheated cell and is denied his medication, his wife said in the letter.
She said he had lost almost seven pounds (3kg) and only gets rice and barley to eat.
"For hours each day, the prosecutors interrogate him, browbeat him, lecture him and berate him, outside the presence of his attorneys, in an effort to extract a confession," Mrs Ghosn claimed.
"No one should be forced to endure what my husband faces every day, particularly in a developed nation like Japan, the third largest economy in the world."
"My husband is well known as a person of unimpeachable honour, honesty and integrity," Mrs Ghosn said.
Mr Ghosn has also been arrested, though not charged, on allegations over the shifting of personal investment losses worth 1.85 billion yen (£13m) to Nissan.
Last week, he insisted he had been "wrongly accused and unfairly detained" as he appeared publicly in court for the first time since his arrest.
Tokyo deputy chief prosecutor Shin Kukimoto said last week that authorities are confident they have a case.
Mr Ghosn led a global alliance between Japan's Nissan and France's Renault - which in recent years also added Mitsubishi - and he remains head of Renault.
He is credited with a spectacular turnaround in Nissan's fortunes over the last two decades.