He spent the night in hospital in Milan for checks, having tested positive on Wednesday.
Earlier his right-wing Forza Italia party said his condition was not a cause for concern, and "he is fine".
The media tycoon's partner and two of his children also tested positive.
Berlusconi
has been in self-isolation with his 30-year-old partner, the Forza
Italia MP Marta Fascina, at his villa in Arcore near Milan, after
holidaying in Sardinia.
The infection rate in Sardinia - famous for its Emerald Coast beaches - is higher than in much of Italy.
"A
small precautionary hospitalisation was needed to monitor the progress
of Covid-19 but he is fine," said Forza Italia senator Licia Ronzulli.
Berlusconi was transferred to an isolation ward at San Raffaele Hospital
on Thursday night. Double pneumonia - inflammation of both lungs - is a
common complication with Covid patients. The disease is generally more
severe among the elderly and those with other medical conditions.
Berlusconi,
a billionaire who used to own AC Milan football team, remains popular
in Italy and he has had many messages of support since being diagnosed.
In
2014 he was ordered to do a year of community service after being
convicted of tax fraud. His business deals have long been under
scrutiny, as have his romantic liaisons, including reports of raunchy "bunga bunga" parties.
Berlusconi
addressed party activists in Genoa by phone on Thursday, saying "I'm
continuing to work as best I can on the current election campaign".
Separately, he said he did not know how he had caught the virus.
Italy
will hold regional elections on 21-22 September, which had to be
postponed at the height of the pandemic in Italy in spring.
Last month Berlusconi hosted businessman Flavio Briatore at his luxury Sardinia residence, Villa Certosa.
Mr
Briatore, who tested positive for coronavirus in late August, owns a
nightclub on the island called Billionaire and used to run the Benetton
Formula One racing team.
Mr Berlusconi served as Italian prime minister four times and last year was elected to the European Parliament.
