Dr Fauci, a member of the White House's coronavirus task force, said the president's unveiling of his judicial nominee was a "superspreader event".
Dozens of White House aides and other contacts were reportedly infected.
President Donald Trump's doctors have just cleared him to hold public events as he himself recovers from Covid-19.
Mr Trump - who was discharged from hospital on Monday after three nights - was asked in an interview with Fox News on Friday evening about the symptoms he had experienced.
The president said he had not felt strong, but had found no problem breathing.
Asked if he had re-tested for coronavirus on Friday, the president said he was at "either the bottom of the scale or free", though he did not say his results were negative.
Mr Trump is expected to host an in-person White House event on Saturday.
"The data speak for themselves - we had a superspreader event in the White House, and it was in a situation where people were crowded together and were not wearing masks."
Dr Fauci also noted experts have been recommending mask-wearing for the last six months, and condemned talk of a coronavirus "cure" - a word Mr Trump has used in reference to the experimental Covid-19 treatments he received during his recent stay at a military hospital.
An event at the White House on Saturday 26 September, for the president's nomination of Amy Coney Barrett as a Supreme Court judge, is thought to be the root of the localised outbreak, as many attendees have since tested positive.
Large gatherings are still banned in the nation's capital due to Covid-19, but federal property like the White House is exempt.