Newly discovered video caught Kyle Rittenhouse fantasizing about being a vigilante gunning down shoplifters — just 15 days before he shot three protesters in Kenosha, Wisconsin, according to prosecutors seeking to use it in his trial.

The 29-second clip included in a court motion filed Wednesday shows a CVS store, with a voice sounding like Rittenhouse suggesting that people running out were shoplifters.

“It looks like one of them has a weapon,” says the person prosecutors identify as Rittenhouse, but who was not actually seen in the video.

“Brah, I wish I had my f—ing AR. l’d start shooting rounds at them,” says the same person in the video used in the filing.

Kenosha County Assistant District Attorney Thomas Binger said his office only obtained the video on Aug. 12. The filing did not reveal how the video emerged, nor why prosecutors believe it is Rittenhouse, who was just 17 at the time.

Binger filed a motion to use the clip in the trial of the teen, who has always insisted he was acting in self-defense when he fatally shot Anthony Huber and Joseph Rosenbaum and injured Gaige Grosskreutz during a Black Lives Matter protest last August.

“The defendant’s state of mind … is a critical issue in this case,” the motion stressed, saying the “video from only 15 days earlier … provides crucial insight.”

“Quite simply, the defendant saw something, jumped to a conclusion based on exactly zero facts, and then threatened to kill someone based on his baseless assumption and wrongful interpretation,” the motion claimed.

“The defendant’s understanding of the proper use of his ‘AR’ and of deadly force is crucial to this case, and this video demonstrates that the defendant was eager to use deadly force in an unlawful situation,” the prosecution motion insisted.

“The video also demonstrates that the defendant fervently sought to insert himself as an armed vigilante into situations that had nothing to do with him,” the motion argued.

It also shows that he “was ready and willing to use deadly force in a situation where it was completely unjustified,” the prosecutors said.

Rittenhouse’s lawyers have yet to respond to the new motion, court records show.

A second motion also seeks the names of anyone who has donated to Rittenhouse’s legal defense or $2 million bail, and even those who bought “Free Kyle” merchandise through his family’s website, to ensure they cannot end up on the jury pool.

Kenosha County Circuit Judge Bruce Schroeder set a hearing for the motions for Sept. 17.

Rittenhouse maintains he fired in self-defense in the fatal shootings caught in graphic videos and images at the fiery protest over the police shooting of Jacob Blake.

His trial is scheduled for Nov. 1 on charges of reckless homicide, recklessly endangering safety, attempted first-degree intentional homicide and being a minor in possession of a dangerous weapon. He faces life in prison if convicted.

With Post wires