Ryan Garcia flunked a drug test the day before his huge upset win over Devin Haney — testing positive for a performance-enhancing substance — but the star boxer is calling BS!

25-year-old Garcia was given a test on April 19 by the Voluntary Anti-Doping Association (VADA), the day before the Haney match, and was popped for Ostarine, according to boxing reporter Mike Coppinger.

The substance Ostarine aids in muscle growth and can increase strength.

Garcia quickly went to social media to adamantly deny the claims.

“Everybody I came on here to address this bull f***ing s**t claim that I cheated,” Garcia said, continuing, “Everybody knows that I don’t cheat.”

“Why didn’t they come out with this before the fight? Why would they let me step into the ring as a cheater and come out with the victory and then post this?” Ryan asked.

Of course, Ryan, who came in over 3 pounds over the 140 lb. threshold, won by majority decision, putting 25-year-old Haney down on the canvas three times during the 12-round fight.

It was hands down the best win of his career … but barring a reversal of course, it’s likely the victory will be overturned, and could be declared a “no contest.”

SI briefly spoke with Devin’s dad and trainer, Bill Haney … who was furious over the news.

“You trying to hurt my baby. We played it fair. We didn’t do it like that. F**k boxing, this is some bulls**t. I came from the streets. You scared of Devin on a level playing field? I feel f**ked up over this. If you can brag about doing it, you shouldn’t be on drugs.”