The wife of ousted Nissan Motor Co. executive Carlos Ghosn urged an international human-rights watchdog to raise awareness of her husband’s treatment at the hands of the Japanese criminal justice system, ahead of a court hearing on whether to grant Mr. Ghosn bail after more than 50 days in jail.

In a nine-page letter to Human Rights Watch made public on Sunday, Carole Ghosn says that her husband has limited opportunities to talk with his lawyers and lost almost seven kilograms, or more than 15 pounds, of weight in two weeks...