Keith Gill, 34, who is known as Roaring Kitty on YouTube and DeepF*****gValue on Reddit, said that he lost more than $13m on Tuesday as share prices plummeted.
He shared the information on Reddit’s WallStreetBets forum, where the tipster has been regularly sharing updates on his investment, with many users refusing to sell their GameStop shares until he does.
Mr Gill, who is known for wearing a bandana in his YouTube videos, has 50,000 shares and 500 call options in the gaming merchandise retailer, which he bought for $53,000 (£38,382) in June 2019.
The value of his stake fell significantly on Tuesday after the share price for GameStop dropped by around 60 per cent, and followed a fall of about $5.2m (£3.8m) on Monday.
Despite the dramatic drop over the first two days of this week, Mr Gill still has a profit of $7.6m (£5.5m) from his initial investment. He has not sold any of his shares in the company.
Following his advice, Reddit users carried out a short squeeze last month and saw the Gamestop share price rise to a high of $483 (£353), before dropping to $90 (£65) on Tuesday.
In an interview with The Wall Street Journal, Mr Gill said: “I didn’t expect this,” and added: “This story is so much bigger than me.”
The squeeze has made headlines worldwide, and caused major market volatility as several trading platforms temporarily halted trades on GameStop and AMC, among others.
In a statement, treasury spokeswoman Alexandra LaManna said: “Secretary Yellen believes the integrity of markets is important and has asked for a discussion of recent volatility in financial markets and whether recent activities are consistent with investor protection and fair and efficient markets.”
Ms Yellen will speak to the heads of the Securities and Exchange Commission (SEC), the Federal Reserve, the Federal Reserve Bank of New York and the Commodity Futures Trading Commission on Thursday.
Last week, the SEC warned that “extreme stock price volatility has the potential to expose investors to rapid and severe losses and undermine market confidence.”
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