The boss of trading firm Polymarket had a meltdown after Donald Trump Jr. joined a rival business, according to the New York Post.
Shayne Coplan, 28, runs the prediction-market giant where users can wager on sports, politics, celebrity gossip, and global events. Sources told the Post he’s locked in an all-consuming rivalry with Tarek Mansour, 30, the CEO of competitor Kalshi, whose net worth is around $2.6 billion.
Coplan has reportedly dismissed Kalshi as a “copycat” of his platform, a jab that stings more given that Kalshi has since eclipsed Polymarket in size. Kalshi also reportedly tipped off federal agents ahead of a November 2024 raid on Coplan’s apartment, part of a probe into whether Polymarket was running illegal gambling operations.
Office life under Coplan can get volatile, sources told the newspaper, especially when Kalshi news breaks. The breaking point came early last year, when word spread that Trump Jr. had signed on as a strategic adviser to Kalshi.
“Coplan screamed so loud that nearly the entire floor was startled,” one employee who witnessed it told the Post.
A source familiar with the situation told the Daily Beast that the frustration stemmed from more underhand tactics from Kalshi, which, the insider said, “knew that [Trump Jr.’s investment firm] 1789 was investing in Polymarket and just beat them to announcing, so the frustration was pretty understandable at the time.”
By August 2025, Trump had also joined Polymarket’s advisory board, a move that coincided with an investment in the company from 1789 Capital, the “anti-woke” investment firm where he’s a partner.
Sources describe Coplan as prone to blowups over mistakes, missed market opportunities, and bad press, allegedly pulling his own hair during the worst of it.
Business-wise, though, the two rivals have quietly made peace. Coplan and Mansour reportedly teamed up to launch a joint venture capital firm aimed at building out infrastructure for the prediction-market industry, an olive branch that sits awkwardly alongside the office horror stories.
The numbers explain some of the tension. Polymarket used to out-trade Kalshi, but that flipped. Kalshi handled $31.5 billion in trades in June alone, dwarfing Polymarket’s $10.6 billion, according to Dune Analytics.
Polymarket has raised roughly $2 billion since launching in 2020 and was valued near $15 billion in its latest funding round. Kalshi has raised about $2.6 billion, hit a $22 billion valuation in May, and is reportedly in talks that could push that number to $40 billion.
The feud comes on the heels of another prediction-market scandal touching President Donald Trump’s inner circle.
Gabriel Perez, who has run Trump’s teleprompter since 2016 and was among the few staffers with advance access to his State of the Union remarks this February, allegedly pocketed more than $100,000 betting on Kalshi using that inside knowledge, sources told ABC News.
White House press secretary Karoline Leavitt confirmed in mid-July that Perez had been placed on unpaid leave, saying Trump himself made the call and “believes it’s deeply unfortunate and frankly a disgrace.”
The White House had already warned staff in March that insider trading is a federal crime, after a wave of suspicious bets tied to major events, including the capture of Venezuelan dictator Nicolas Maduro, were allegedly placed by insiders, one of them a soldier involved in the operation itself.
