Here's the deal ... Barstool Sports posted a pic on Instagram showing a hippo chewin' on another hippo's butt and included the caption, "As soon as bae gets out of the shower #AssEatinSZN."

Nothing out of the ordinary there ... until we looked at the comments and noticed Tom Brady weighing in -- with some laughing face emojis and the word, "Yep."

GISELE!!!!!!

For the uninitiated, Ass Eatin' Season is a thing on Barstool. And, by the way, they even tweeted about it already, saying ... "Tom Brady is about that life #AssEatinSZN."

Last we saw Tom and Gisele, they were at a U2 concert together in Boston and Tom was belting out the lyrics at the top of his lungs.