Jill Zarin's mother, Gloria Kamen, is dead.

The former 'Real Housewives' star announced her mom's death in a heartfelt social media post Tuesday ... saying Gloria died earlier this morning.

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The cause and manner of death are unclear.

Jill shared 20 photos of her mom in her Instagram post, remembering Gloria as "my best friend, my mentor, and the most important person in my life." 

She says Gloria "was the real OG housewife who raised incredible women and touched the lives of millions with her counsel and kindness" ... thanks to the book they wrote together, "Secrets of a Jewish Mother."

Jill finished off her post by adding, "Thank you mommy for everything you did for me and giving me the most beautiful family. You picked the best father I could ever ask for and you were always right about everything even if it took me a while to see it."

Several other 'Real Housewives' alums -- including Bethenny Frankel, Marysol Patton and more -- shared their condolences in the comments.

Gloria was 91.

RIP