In a new excerpt from RW's upcoming book -- obtained by Us Weekly -- the actress reveals the Grammy winner likely isn't her biggest fan ... as the two were juxtaposed quite a bit over the years.

While Rebel admits she has never confronted Adele directly about the alleged hate, she does say Adele would avoid her like the plague at industry events … which seems to be why she thinks Adele doesn’t like her.

She adds ... "As if my fatness might rub off on her if I were near her for more than thirty seconds."

Rebel claims the hitmaker also disliked being compared to her character in the "Pitch Perfect" franchise ... who was infamously named "Fat Amy."

Remember, Rebel and Adele have both gone through impressive weight loss transformations in recent years … and Adele, in particular, has been both praised and criticized for it, for various reasons.

Adele isn't the only celeb to be name-dropped in Rebel's forthcoming memoir. Remember, she called out Sacha Baron Cohen in one controversial chapter … making allegations against him, and labeling him an “a**hole.”

There’s been a lot of back and forth between those two this past week, but Sacha has outright denied her characterization of their work together.