Apple and Samsung were fined €10m and €5m apiece following investigations that determined they held "unfair commercial practices".
In addition, the agency stung Apple with a further €5 million fine over complaints it failed to provide end-users with clear information about how to maintain and replace failing batteries.
The authority said the companies were given the "maximum prescribed fines" due to their size and the seriousness of the allegations brought against them.
In a statement to the Washington Post, Samsung said it was disappointed by the Italian group's ruling.
CBI vs CBI: Opposition attacks govt over removal of Verma
This is precisely to secure the independence of the CBI, but the same has been violated by these orders, it said. DMK chief M K Stalin questioned if this move was initiated to cover up the Rafale deal investigations.
Oil extends declines as Saudi Arabia commits to meet demand
The killing of journalist and US resident Jamal Khashoggi by Saudi agents has stirred calls for USA sanctions on the kingdom. The pledge comes despite Saudi Arabia's previous threats to retaliate to any sanctions with "greater action".
US warships sail through Taiwan Strait
Beijing, however, regards democratic Taiwan to be a breakaway province still within the umbrella of its global foreign policy. Despite this, Washington has kept informal relations with the island nation after severing diplomatic ties with it in 1979.
The anti-trust watchdog said Samsung "insistently proposed, from May 2016, to consumers who had purchased a Note 4 (placed on the market in September 2014) to proceed to install the new Android firmware called Marshmallow prepared for the new model phone Note 7, without informing of serious malfunctions due to the greater stresses of the hardware and requiring a high fix cost for the out-of-warranty repairs connected to such malfunctions ". Apple has apologized and it cut the costs of battery replacement for iPhone 6 and 6s models since.
At first, Apple had denied it intentionally shortened the life on any of its products.
"The two companies have induced consumers to install software updates that are not adequately supported by their devices, without adequately informing them, nor providing them with an effective way to recover the full functionality of their devices", the AGCM said. According to the statement, Apple pushed users to update to an iOS version which was "made for iPhone 7". Apple has also faced questions from the US senate over the issue, as well as more than 60 separate lawsuits, which have been ordered to be consolidated into a single suit in the Northern District of California.
The authority demanded that both the companies display a notice on their Italian websites informing the users about the decision. Samsung was fine after an investigation showed that the company released an update for the Galaxy Note 4 that was designed for the Galaxy Note 7, making the device sluggish.