The princess turned 49-years-old on Sunday, and an official statement thanked the Japanese public for their support during her decade-long battle with adjustment disorder.
Biographer Ben Hills has told Radio Australia's Connect Asia program he disagrees with that diagnosis.
"It's quite clearly not that, if you look adjustment disorder up in the psychiatric bible, you'll find that it's a mild form of mental illness that always resolves itself in six months," he said.
"It's not six months, it's nine years since Princess Masako first showed symptoms, and I don't think anyone is in doubt now that what she's got is the most serious case of depression."
‘Best and brightest’
Before marrying the Crown Prince, Princess Masako was known for her intellect and professional achievements.
"She speaks half a dozen languages, she has degrees from both Cambridge and Harvard, she's a high-flyer and may have been the first woman ever to head a Japanese Government department in Tokyo, so she really is the best and brightest of her generation," said Mr Hills.
"The marriage was really not something that she wanted, I think she was pressured into it, she turned the crown prince down a couple of times, but eventually she succumbed, and since then I'm afraid it's all been downhill for her."
"She finds herself a prisoner in a gilded cage, her entire existence has been negated, and this has caused her to plunge into the most serious mental illness."
"She's not allowed to leave [the palace] without permission...she has no credit card, she doesn't have unrestricted access to the phone, she doesn't have a passport and she doesn't even actually have a name, so her entire existence has been negated."
Ben Hills says Princess Masako could have been an asset for Japan.
"She could have been the greatest thing to happen to Japan in donkeys’ years," he said.
"I mean, here she is, a bright, articulate, modern, multi-lingual woman, she could have been the best ambassador Japan ever could have imagined."
"Instead they pummelled and forced her into this mould where she dutifully trots along three steps behind her husband, doesn't speak unless she's spoken to, gives the little smile, gives the little wave, what a terrible waste of talent!"
Ben Hill is a former Japan Correspondent for Fairfax media and the author of Princess Masako: Prisoner of the Chrysanthemum Throne.