"Our estimate is that up to now, 25 million Iranians have been infected with this virus," Rouhani said during a televised meeting of Iran’s virus-fighting taskforce.
"We have to consider the possibility that 30 to 35 million more may face infection," he added, citing the results of a study by the Health Ministry.
"We have not yet achieved herd immunity and we have no choice but to be united and break the chain of transmission of the coronavirus."
Rouhani said more than 200,000 people had been hospitalized and that Iran may have to prepare itself for double the amount of hospitalizations it has had in the past five months, according to the study.
The study shows that "out of every 1,000 infected people, 500 show no symptoms" Rouhani added, warning that they present the main challenge by "spreading more virus and over a longer period" than those identified.
He urged all Iranians to join hands and break the chain of the outbreak by observing health guidelines.
“Victory over the disease is possible, but it may be a little difficult and time consuming. We will definitely win and we must stand firm and know that victory is certain,” Rouhani emphasized.
The virus has killed more than 588,000 people and infected nearly 14 million around the world since first being detected in China late last year.
Iran has been battling a resurgence of COVID-19, with figures showing a rise in both new infections and deaths since a two-month low in May.
On Saturday, the Health Ministry reported another 188 coronavirus deaths and 2,166 cases of infection in the past 24 hours.
That took the overall toll to 13,979 dead out of 271,606 cases since Iran reported its first cases in mid-February.
The rising toll has prompted authorities to reimpose restrictions in worst-hit provinces after being lifted country-wide in April.
Authorities on Saturday reintroduced one-week restrictions in the capital Tehran including banning religious and cultural functions, closing boarding schools, cafes, indoor pools, amusement parks and zoos.
From Sunday, 22 cities and towns in the southwestern Khuzestan province will be under a three-day lockdown, the province's governorate announced on Saturday.
Iran, with a population of more than 80 million, has been the West Asian country hardest hit by the epidemic since announcing its first cases in mid-February, with infections and deaths rising sharply since restrictions were eased beginning in mid-April.
Iran has refrained from imposing full lockdowns but closed schools, cancelled public gatherings and banned travel between provinces in March, before lifting the restrictions the next month to reopen its sanctions-hit economy.
AFP, Reuters and Press TV contributed to this story.