Though he didn't go into specifics, Fisher also said Whedon's alleged behavior was "enabled in my ways," by producers Geoff Johns and Jon Berg.
"Accountability>Entertainment," added Fisher.
Reps for Whedon, Johns and Warner Bros., the studio behind "Justice League," weren't immediately available for comment. Whedon had no comment when contacted by Variety.
Nia DaCosta, who directed the upcoming "Candyman" flick, showed support to Fisher on Twitter -- and said she was "very proud" of him for speaking out.
"The threat of retribution is so real which is in part how abusive and unprofessional behavior is perpetuated in our industry," she added. "Speaking up absolutely comes with consequences so I salute his bravery."
Fisher's tweet comes just days after another post in which he criticized Whedon. Sharing a clip of him saying complimentary things about the director while promoting the movie at Comic-Con 2017, he added, "I'd like to take a moment to forcefully retract every bit of this statement."
In the footage, he called Whedon a "great guy" who Snyder picked to "clean up and finish up" the movie.
"Justice League" was released in 2018, but the final product wasn't Snyder's full vision. When the director stepped away from the project during post-production following the death of his daughter, Whedon was brought in to finish the film and handle reshoots.
When the film was released, viewers weren't exactly satisfied by what they saw, leading to an extremely vocal subset of the fandom demanding the release of what has become known as the "Snyder Cut." It's believed footage of Cyborg was trimmed down in Joss' version.