Christopher Nolan says a Peloton workout turned into a scathing, live movie review ... as the instructor -- who probably didn't know he was watching -- unloaded on his film during class!!!

The "Oppenheimer" director won a New York Film Critics Circle award Thursday for best director, and during his acceptance speech he recalled the time a Peloton instructor took a shot at his work.

Christopher says he was in a virtual Peloton class when the woman leading the workout, Jenn Sherman, started talking trash about "Tenet" ... asking if anyone else saw or understood the 2020 movie. As Jenn put it, "I'm not kidding. What the f*** was going on in that movie?" She also groused she'd wasted hours of her life that she'd never get back.

BTW, he didn't call out Jenn by name, but internet sleuths quickly found the clip.

CN says he was also getting his butt kicked in the workout when she piled on, but it sounds like she had no idea Christopher was in the class.

The funny anecdote was part of Christopher's broader point -- that everyone's a critic these days.