The island, in the Gulf of Naples, was engulfed by heavy rain overnight, with the landslide hitting the hamlet of Casamicciola Terme early on Saturday morning, flooding homes and sweeping away several car
The deputy prime minister, Matteo Salvini, had earlier said that eight people had died. But by Saturday evening there was one confirmed death with 10 people still missing.
The first victim was confirmed to be a woman whose body was found by rescuers, according to a media briefing by the prefect of Naples, Claudio Palomba.
The interior minister, Matteo Piantedosi, had earlier warned there were people trapped in the mud, saying it was a “very serious” situation.
Electricity has been cut off in the affected area and about 30 families have been stranded in their homes in the hamlet of Lacco Ameno, where several buildings are reported to have collapsed.
"94% of municipalities are at risk of flooding, landslides, coastal erosion," Fabrizio Curcio told La Stampa newspaper, adding that "all of Italy is at risk."
Known for its natural hot springs, Ischia has a population of about 20,000 and is popular with Italian and foreign tourists. Ferries to and from the island have been suspended.
Ischia was at greater risk because many of its houses were built illegally, but stressed that other parts of Italy with fewer land planning violations were also vulnerable.
“It’s a tragedy, the number of missing people in Casamicciola is still uncertain,” said Ferrandino earlier on Saturday.