The former First Lady went on Fox News and was asked if she could share anything about Donald Trump that people don't know ... and her answer was interesting.

Melania says DT is a super great guy ... describing him as warm and fun ... two adjectives lots of folks don't necessarily associate with our former Commander in Chief.

A quick scan of Trump's social media presence doesn't really give off the warm or fun vibe either ... but, Melania's saying that's what he's really like behind closed doors.

Donald's third wife says he's just a passionate guy who wants to make America safe ... and Melania feels he treats everyone equally, man or woman, and doesn't discriminate when he needs to get tough with somebody.

In other words, she's describing Trump in the exact opposite way of a lot of his haters.

Melania's promoting her memoir, titled "Melania" ... which she says is packed with other "cute" and "nice" stories about Donald.