Sanctioning body the WBC confirmed it had “received notification” from the Voluntary Anti-Doping Association (Vada) that Garcia returned “adverse findings” in tests taken in April.
ESPN has reported that it obtained a letter, external from Vada showing Garcia’s positive result for ostarine.
The samples were allegedly taken the day before and the day of his win over Haney in April, but the results came through afterwards.
“Everybody knows that I don’t cheat,” Garcia, 25, wrote on X.
“Never taken a steroid, I don’t even know where to get steroids. I barely take supplements. Big lies, I beat him.”
Garcia has 10 days to request for his B sample to be tested.
BBC Sport has contacted Vada and the New York State Athletic Commission for comment.
Ostarine, which is used to support muscle growth, has been on the World Anti-Doping Agency’s banned substance list since 2008.
Garcia floored Haney three times in his upset victory, but did not win his American compatriot’s WBC light-welterweight title after missing weight for the contest.
The WBC requires all its fighters to enrol in its ‘clean boxing program’ and employs Vada to oversee the drug testing.
Golden Boy Promotions, Garcia’s promoters, said they will “determine how this finding came to be”.
“Ryan has put out multiple statements denying knowingly using any banned substances – and we believe him,” the statement added.
Haney told ESPN the result has “put the fight in a completely different light”.
“We learned about this situation not too long ago and it’s unfortunate Ryan cheated and disrespected both the fans and the sport of boxing by fighting dirty,” Haney said.
“Ryan owes the fans an apology, and by his recent tweet he still thinks this is a joke. We put our lives on the line to entertain people for a living.”
British boxer Amir Khan was handed a two-year ban by UK Anti-Doping after he tested positive for ostarine in 2022.