Tiziano Renzi and Laura Bovoli, suspected of issuing false invoices, were put under house arrest to avoid the destruction or manipulation of evidence, press reports said, quoting judicial sources.
Renzi, who headed the Italian government from February 2014 to December 2016, said his family was being "smeared" because of his political activities.
Renzi described the arrest as "abnormal" and said that, while he wouldn't say his family was the victim of a plot, the news was a "media masterpiece" as it had overshadowed the vote by Five-Star Movement members against Deputy Prime Minister Matteo Salvini's parliamentary immunity being lifted in a potential migrant kidnapping case.
"Those who have a minimum of legal knowledge know that depriving someone of their personal liberty for something like this is abnormal," Renzi, a PD Senator, reportedly said.
"What's more, those who know how things really are know that the documents do not say the truth. But that will be for the trial."
"I have a lot of faith in the Italian justice system," Renzi, 44, wrote in a post on Facebook. "And so I am anxious to attend the trial."
Tiziano Renzi (R) and his wife Laura Bovoli, parents of former Italian Prime Minister Matteo Renzi, received the house arrest notice on Monday. Photo: Carlo Bressan/AFP
“People who have read the (investigative) papers assure me they have never seen such an absurd or disproportionate measure,” Renzi wrote, referring to the house arrest.
Tiziano Renzi is a businessman and was previously a local politician in the town of Rignano sull’Arno, near Florence.
He has faced a string of legal problems since his son became prime minister in 2014, causing embarrassment to the center-left leader and handing ammunition to his political opponents.
Renzi left office in December 2016. Last week he published a book about his time as prime minister, and was in Turin on Monday for a launch event when news of his parents' house arrest broke.