Posobiec has promoted many falsehoods,[28] leading to Philadelphia calling him the "King of Fake News" in 2017.[9] He was one of the most prominent promoters on social media of the Pizzagate conspiracy theory, which falsely claimed that high-ranking officials were involved in a child-sex ring centered at a Washington, D.C. pizzeria.[9][16] He live-streamed an investigation of the pizzeria and was asked to leave after attempting to broadcast a child's birthday party being held in a back room.[54] Posobiec later said he had always thought the Pizzagate theory was "stupid" and had filmed his visit to debunk it.[9]

Posobiec attempted to discredit anti-Trump protesters in November 2016 by planting a sign at a protest reading "Rape Melania".[55][56][57] Posobiec denied his involvement to BuzzFeed News, but the same phone number was used in his contact with the website and the text messages he reputedly sent.[58] He said he had been questioned about it by the Secret Service.[9] Posobiec organized the DeploraBall, an event held on January 19, 2017, to celebrate Trump's inauguration.[59]

In December 2016, Posobiec claimed without evidence that Disney had re-written scenes in the Star Wars movie Rogue One to add "Anti Trump scenes calling him a racist", and called for a boycott of the Star Wars franchise. Disney denied the allegations.[60]

Posobiec falsely said that former FBI director James Comey, at a United States Senate hearing on May 17, 2017, "said under oath that Trump did not ask him to halt any investigation". The claim was later repeated by conservative personalities and media outlets, including Fox News, Rush Limbaugh and the InfoWars website.[27] Posobiec promoted the discredited conspiracy theory that Seth Rich had leaked e-mails from the Democratic National Committee to WikiLeaks.[22] Posobiec promoted a hoax that CNN had published and then deleted an article defending Bill Maher's use of a racial slur.[61]

In June 2017, shortly after Republican congressman Steve Scalise was shot and injured during a baseball practice, along with four others, Posobiec tweeted that it was a terrorist attack and blamed comments from liberal anti-Trump individuals. Later, he falsely tweeted that former United States Attorney General Loretta Lynch had called for "blood in the streets" the previous March[62] and that Bernie Sanders had ordered his followers to "take down" Trump.[63]

In December 2017, Posobiec, along with Cernovich, The Gateway Pundit, and InfoWars, promoted a false theory that a passenger train derailment near Dupont, Washington, was linked to the Antifa anti-fascism movement.[64]

In October 2019, after Lt. Col. Alexander Vindman, a White House national security official and decorated Iraq war veteran, testified in Congress about President Trump requesting that the Ukrainian President investigate his political rival Joe Biden, Posobiec falsely claimed that Vindman had been advising the Ukrainian government on ways to prevent Trump from implementing his foreign policy goals.[65]

In June 2020, during the protests against racism and police brutality in the wake of the murder of George Floyd, Posobiec falsely claimed that there were pipe bombs planted at the Korean War Veterans Memorial in Washington D.C. and that "federal assets [were] in pursuit". There were no pipe bombs nor was there any evidence that any "federal assets" investigated. The claim was, however picked up by The Gateway Pundit and retweeted by over 29,000 users on Twitter.[66]