It works by encouraging operating system updates for older phones which slow them down, thereby encouraging the purchase of new phones.
In a statement the antitrust watchdog said "Apple and Samsung implemented dishonest commercial practices" and that operating system updates "caused serious malfunctions and significantly reduced performance, thus accelerating phones' substitution".
In a statement on Wednesday, the Italian competition authority, the Autorità Garante della Concorrenza e del Mercato (AGCM), said both companies had violated consumer protection laws by "inducing customers to install updates on devices that are not able to adequately support them".
The Italian antitrust authority fined the companies 10 million Euros (£8.84m) and 5 million Euros (£4.4m).
Samsung "insistently suggested" to owners of its 2014 Note 4 phone to install a new version of Google's Android operating system intended for the more recent Note 7, the ACGM said.
This is the first time that Samsung has been questioned over the software updates for devices.
Explanations for the slowness range from Apple's iOS causing problems on older devices to the firm deliberately slowing down old phones to make people buy the new handsets.
Jordan cancels 2 annexes from peace treaty with Israel
Activists have been asking the government not to renew the agreement and to annul Israel's ownership of Jordanian lands. But Safadi said Israel had made no effort to enter such negotiations, and even if it did, Jordan would not budge.
Apple previously admitted to the practice of slowing down phones, but the company stated that it was to improve the battery life of older models and increase the device's lifespan.
Apple got hit with a €5 million fine for a similar problem.
Samsung plans to appeal the fine. The authority also argued that Apple should have issued instructions about replacing the battery in the iPhone.
The watchdog also added that both Apple and Samsung had not provided their clients with enough information on the impact of the new software "or any means of restoring the original functionality of the products".
Apple actually admitted a year ago that it slowed phones down with software, though the company claimed that it was doing it to prevent the phone shutting down unexpectedly due to degradation of their lithium batteries (there's that extra €5m, by the way).
Teenagers posing with their Samsung Galaxy S4 (left) and iPhone 4 smartphones. Interestingly, both companies were fined €5m but Apple was slapped with an additional €5m fine for not letting users know about the nature of lithium batteries-how long they last and in what time frame they degrade.