The actress's latest revelation comes courtesy of her People Mag interview ... which just dropped this nugget of news Saturday. In their sit-down with her ahead of Jada's new book, they have her quoted as talking about Tupac's baldness -- with her saying he had alopecia.

Jada says, "I don't think Tupac ever talked about the alopecia he suffered from," going on to explain that his hair started falling out around 1991 -- when he would've been about 20 years old -- shortly after he had a famous run-in with cops in NorCal.

She adds, "[H]is alopecia patterns were far more extreme than mine. I don't think Pac ever talked about his alopecia, but he also looked really good with a bald head. But that was during a time and during an era that you wouldn't -- he just wouldn't talk about it."

One thing she does firmly believe -- if Pac was alive today ... he'd probably talk about it openly.

This is fascinating for a couple reasons ... for starters, it would seem to explain Tupac's bald look in the latter years of his life, when he was most famous. While he was younger and on the rise, he rocked notable 'dos -- particularly in the film "Juice."

It's also interesting because alopecia was apparently at the center of the whole Oscars slap last year -- when Chris Rock cracked a joke about Jada, spurring Will Smith to get up there and slap him.

Alopecia proved to be a touchy subject, and even now ... Jada thinks Chris took a low blow by referring to her condition. Still, Jada says she couldn't believe Will did what he did ... especially when he called her his "wife," a term she says they hadn't used in years.

BTW, this new Tupac news follows yet another big headline from earlier in the week -- namely, Jada calling the late rapper her "soulmate" ... but insisting they never banged.