The billionaire media tycoon, who made his fortune from commercial television, has suffered repeated bouts of ill-health in recent years and came out of hospital just last week.
"He has been admitted to intensive care because a problem caused by an infection has not been resolved but he is speaking," Italy's Foreign Minister Antonio Tajani, a long-time ally of Berlusconi, told reporters in Brussels.
A source at the San Raffaele hospital confirmed that Berlusconi was being treated in the intensive care section of its cardiothoracic wing and would remain in hospital overnight.
Italian media reports said Berlusconi was taken to hospital after complaining of breathing difficulties.
His eldest daughter Marina Berlusconi and his brother Paolo Berlusconi visited him during the day, Italy's ANSA news agency reported.
Berlusconi's Forza Italia party is part of Prime Minister Giorgia Meloni's right-wing coalition, although he does not have a role in government.
Meloni sent a goodwill message on social media.
"Sincere and affectionate wishes for a speedy recovery to Silvio Berlusconi," she said, adding "Forza Silvio (Come on, Silvio).
Berlusconi built Italy's biggest commercial TV network and gained an international profile as owner of European soccer champions AC Milan before entering politics in 1994 when the previous political class was brought down by a corruption scandal.
His health has deteriorated in recent years. He had heart surgery in 2016, has also had prostate cancer, and has been repeatedly admitted to hospital over the past couple of years after contracting COVID-19 in 2020.
Berlusconi stepped down as prime minister for the last time in 2011 as Italy came close to a Greek-style debt crisis and weighed down by his own scandals, including his notorious "bunga bunga" parties.
He was returned to the Senate (upper house) of the Italian parliament after a general election last September.
Berlusconi has recently stirred controversy with his criticism of Ukraine's President Volodymir Zelenskiy, putting him at odds with Meloni.
An Italian court acquitted Berlusconi in February over allegations of paying witnesses to lie in an underage prostitution case that has dogged the former prime minister for more than a decade.
Berlusconi was accused of bribing 24 people, mostly young female guests at his "bunga bunga" parties, in a previous trial where he was charged with paying for sex with a 17-year-old Moroccan nightclub dancer.
Berlusconi's Fininvest family holding group retains control of the MediaForEurope (MFEB.MI) television business and its shares rose on Wednesday on speculation about potential M&A activity in a post-Berlusconi era.