Oh hell no! -- that's basically what Mya said to a fan who asked about the rumor that the singer had had decade-long affair with Jay Z.

On Thursday an eloquent fan wrote on the "Lady Marmalade" songstress' Instagram, "Heard you and Jay Z had an 11-year-affair. Are you his side chick or nah?"

Instead of turning the other cheek, Mya met the rumor head on, writing back, "Never did, never was, never will."

Mya added, "Illegitimate, thirsty sources with no facts and that's the ‘media' for you-especially today."

There you have it, straight from the rumored mistress' mouth!

We hadn't heard about the Mya sex scandal -- but it sounds like Jay's cheating rumors have been everywhere in recent days so we aren't surprised. Last month Life & Style reported that "Young Forever" rapper had cheated on his wife of six years with Rihanna, Rita Ora, Rachel Roy and the former Miss Belgium.

But if you look at Bey's Tumblr -- all looks well with the power couple -- and we know that Instagram is what really matters ...