Delpopolo, 23, who finished seventh in the 73kg event, accepted his expulsion.
"My positive test was caused by my inadvertent consumption of food that I did not realize had been baked with marijuana, before I left for the Olympic Games," said Delpopolo, who trains in New York.
"I apologize to the U.S. Olympic Committee, to my teammates, and to my fans, and I am embarrassed by this mistake. I look forward to representing my country in the future, and will re-dedicate myself to being the best judo athlete that I can be," he said in a statement.
The United States Olympic Committee told Reuters the food Delpopolo had eaten was a brownie containing marijuana.
The athlete waived the right for his case to be heard before the Disciplinary Commission, and the USOC said it fully supported the expulsion.
"The USOC is absolutely committed to clean competition and stringent anti-doping penalties. Any positive test, for any banned substance, comes with the appropriate consequences and we absolutely support the disqualification. We look forward to witnessing the continued success of our athletes and commend their dedication to clean sport."
Cannabis's place on the World Anti-Doping Agency's (WADA) prohibited list has sparked much debate.
President John Fahey indicated earlier this year that WADA may look at changing the criteria for cannabis as a banned substance for athletes, but no decision is expected this year.
At the moment, a substance appears on the banned list if it meets the following criteria: it is proven to be performance enhancing, it goes against the spirit of sport, or it is dangerous to the health of athletes.
Marijuana, or cannabis, qualifies as a forbidden drug under the current rules, with athletes facing a two-year ban if it is found in their system.
While it is generally accepted that cannabis is unlikely to give athletes a performance advantage in fast-paced sports, some experts say it could prove helpful in sports like shooting or golf where a steady hand is needed.
Delpopolo is set to leave London for the United States on Tuesday.
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