Blake Lively's 'It Ends With Us' Costar Shows Support in Baldoni War

The "It Ends With Us" cast is now publicly choosing sides in Blake Lively's sexual harassment battle with Justin Baldoni, the film's director.

Brandon Sklenar, who plays Atlas Corrigan in the film, encouraged fans to dig into the full complaint Blake filed against Baldoni ... telling his Instagram followers, "For the love of god read this."

He made it clear he's got Blake's back by tagging her in the IG story, and adding a red heart emoji.

It's unclear if Brandon -- who also stars in the "Yellowstone" prequel "1923" -- was aware of Blake's issues with Baldoni while they were shooting the film.

TMZ broke the story ... Blake claims, in legal docs, Baldoni created a hostile work environment on the 'It Ends' set ... showing her lurid videos of his wife, discussing his sex life and more. Her complaint also includes text messages between Baldoni's PR team, discussing a smear campaign against Blake.

Blake believes that campaign to publicly take her down was retaliation for the fact she called for an "all-hands" meeting during filming to openly discuss Baldoni's alleged behavior.

Of course, it's no secret Blake and Justin were not getting along when the movie came out earlier this year. They never posed together at the premiere, and did not promote the film together.

At the time, that was largely believed to be based on creative differences in the direction of the film ... something Brandon addressed back then, saying, "Vilifying the women who put so much of their heart and soul into making this film because they believe so strongly in its message seems counterproductive and detracts from what this film is about."

However, Blake's filing alleges the feud with Justin was rooted in something more sinister than that.

Baldoni's attorney Bryan Freedman has said, “It is shameful that Ms. Lively and her representatives would make such serious and categorically false accusations. These claims are completely false, outrageous and intentionally salacious with an intent to publicly hurt and rehash a narrative in the media.”

In addition to Brandon's support, Blake's gotten backing from several other friends and actors, some of whom weren't involved in the film, like Amber Heard -- who has personal history with Baldoni's embattled PR team -- and Michele Morrone, who did costar with Blake in "A Simple Favor 2."

In a video post, Michele said, “So it’s usually not my thing to make these types of videos, but I think it’s time to stand up for a person I really love, and this person is Blake Lively."