Apple came clean on the practice two days after it was written about by Geekbench founder John Poole in December 2017.
The company later offered replacement batteries to those affected at the discounted price of US$29.
But Samsung has previously said it was not slowing down older devices.
Italy's anti-trust authority said: "As a result of consumer reports and pre-trial activity carried out by the Authority, the Authority decided to initiate two separate proceedings for unfair commercial practices against the companies of the Samsung group and the Apple group operating in Italy."
It said such behaviour could violate a number of articles of the Consumer Code.