The president's son appeared on "Fox & Friends" on Wednesday to discuss the RNC. He called attention to his appearance - "glassy" eyes and a sweaty brow - during the convention and the rumours it sparked.
"I guess there must have been something with the lighting," Mr Trump Jr said. "They started doing this trending thing - 'Donald Trump Jr's on cocaine' and all of that - I said, no no no. You have me confused with Hunter Biden."
Hunter Biden is Democratic presidential nominee Joe Biden's son, and has used cocaine in the past. Hunter Biden was discharged from the US Navy Reserve in 2013 for failing a drug test.
Mr Trump Jr said the claim's were "ridiculous."
"When they can't attack the delivery, when they can't attack the substance, they gotta attack something," he said.