"Most of us feel that the number of deaths are likely higher than that [official] number... I don't know exactly what percent higher, but almost certainly it's higher," Fauci said.
He warned senators against opening up the country too soon, adding that otherwise the virus will spread.
Questioned about the possibility of a new wave of the outbreak in autumn, Dr Fauci said one is "entirely conceivable and possible".
He remotely testified to the US Senate Health, Education, Labour and Pensions (HELP) Committee.
Last week, Fauci announced that he would start a "modified quarantine" after having a "low risk" contact with a White House staffer who had tested positive for COVID-19.
As of 12 May, the total number of confirmed COVID-19 cases in the US stands at 1,351,200, with 80,897 fatalities, according to Johns Hopkins University.