Jason Aldean WARNED 'Idol' Kissing Partner -- I'm Divorcing My Wife

Right before Jason Aldean filed for divorce ... the country star called the "American Idol" hottie he kissed behind his wife's back and warned her about the media firestorm to come ... TMZ has learned.

Sources connected to Brittany Kerr tell us ... Aldean placed the courtesy phone call himself last week ... and politely warned the singer that he was pulling the trigger on his marriage to his high school sweetheart Jessica Ussery.

Translation -- the media's gonna be all over your ass again ... so, govern yourself accordingly.

We're told Kerr appreciated the heads-up ... considering she was Aldean's kissing partner on the night he cheated on his wife back in Sept.

Since the warning, Brittany's been laying low -- shutting down her social media pages and staying out of the public eye.

It's not clear whether Aldean and Kerr have communicated since … or whether their relationship has a future.