London-based Navinder Singh Sarao, 37, must now be extradited to the US within 28 days to face charges that he contributed to the May 2010 crash, when the Dow Jones share index in New York briefly fell by more than 1,000 points.
At the High Court, Lord Justice Gross said it was "clear" the court must turn down his application against extradition.
American prosecutors allege Sarao made millions of dollars through online tradesfrom his parents' west London home which amounted tomarket manipulation and caused the 1,000-point fall on the Dow Jones index on May 6, 2010.
The 22 charges he faces include fraud and "spoofing" - the practice of buying or selling with the intent to cancel the transaction before execution.
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He has previously told the court that he was simply "being good at my job".
The US justice department claims Sarao and his company NavSaraoFutures Limited made 878,000 dollars of profit from the flash crash and a total of 26 million pounds illegally over five years.
The former bank worker and Brunel University student, who lives and worked out of his parents' home in Hounslow near Heathrow airport, is accused of using an automated trading program to "spoof" markets.
During the course of hearings last year, it emerged that he suffers from Asperger's syndrome, a form of autism.
The so-called "flashcrash" saw the Dow Jones industrial average briefly plunge more than 1,000 points on May 6, 2010, temporarily wiping out nearly one trillion dollars in market value.
He was freed in September last year after US authorities agreed he could be released on bail of 50,000 pounds.
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