Martha Stewart sounds like she's got a pretty informal relationship with King Charles III ... she says he's just "Chuck" to her.

We got Martha on her way out of The Whitby Hotel in New York City on Wednesday and our photog asked her if she was heading out to see King Charles and Queen Camilla.

Martha seemed confused about this Charles character ... saying she calls the King, "Chuck."

Chuck or King Charles ... Martha says the guy looks great these days ... ditto for his Royal wife.

Martha is attending a charity event put on by the King ... Charles founded the King's Trust charity five decades ago, and tonight is its annual Global Gala at Christie's New York.

We also asked Martha if she will be giving Chuck and Camilla any home decor tips ... but it sounds like she wants to pick their brains instead.