Floyd Mayweather has denied he broke any rules by having an infusion of saline and vitamins on the eve of his super-fight with Manny Pacquiao.
Usada countered in a statement that the report was "riddled with significant inaccuracies and misrepresentations".
Mayweather said he was "very proud to be a clean athlete".
The 38-year-old added: "I follow and have always followed the rules of Nevada and Usada, the gold standard of drug testing.
"Let's not forget that I was the one six years ago who insisted on elevating the level of drug testing for all my fights."
The report alleges Mayweather breached World Anti-Doping Agency (Wada) rules by having the IV infusion, even though the substances in question were not banned. The report maintains that an infusion "can dilute or mask the presence of another substance that is already in the recipient's system".
It added that the infusion had come to light after Usada agents visited the boxer at his Las Vegas home on 1 May to conduct an unannounced test.
"We believe it is important to immediately correct the false suggestion that Floyd Mayweather violated the rules by receiving an IV infusion of saline and vitamins," said Usada.
"It is simply absurd to suggest that we would ever compromise our integrity for any sport or athlete.''
Usada said Mayweather, who takes on Andre Berto in Las Vegas on Saturday in what he says will be his last fight, had applied for and was granted a Therapeutic Use Exemption by the agency for the infusion, in accordance with regulations set down by the Nevada State Athletic Commission (NSAC).
The agency said although the application was not approved until after the fight, Mayweather "did disclose the infusion to Usada in advance of the IV being administered to him".
"Mr Mayweather's use of the IV was not prohibited under the NSAC rules at that time and would not be a violation of the NSAC rules today," Usada's statement added.
Pacquiao was refused an anti-inflammatory shoulder injection before the same fight - the richest in the history of the sport - after failing to notify the correct authorities.
He later blamed the injury for his points defeat, claiming it stopped him using his right hand.