Sunk into the square, the steps down to the store form an amphitheatre with the curtain of water providing a backdrop.
The eight-metre high glass walls contain a water feature that was, according to architects, supposed to be an "immersive recreation" of running through a fountain as a child.
"The fountain is an expression of child-like excitement that speaks to each one of us," said Stefan Behling, head of studio at Foster + Partners.
"In its simplicity, it echoes the idea of walking into a big fountain without getting wet, and the joy of being alive."
Apple Piazza Liberty is located off the Corso Vittorio Emanuele, and is the tech giant's first Italian outpost in its new design style.
"To work within one of Italy's historic piazzas is both a great responsibility and wonderful challenge," said Jony Ive, Apple's chief design officer.
"We combined two fundamental elements of the Italian piazza — water and stone — adding a glass portal that creates a multi-sensory experience as visitors enter the store through a cascading fountain that seems to envelop them."
Over 20 Gleditsia Sunburst trees have been planted around the square at street level and in planters in the sunken shop.
Beola Grigia, a grey stone quarried in Lombardy and used in much of Milan's architecture, has been used to clad the piazza and walls of the shop's interior.
A floating cantilevered staircase of stone and polished stainless-steel treads leads down into the store. Inside the subterranean Apple Store is a single space, lit from above through skylights supported by backlit ceiling panels.
At the base of the fountain the new outdoor space will be used as an extension for the store's activities.
The piazza will be open 24 hours a day and will play host to a range of cultural events, including a month-long programme in September called Milan Series, where 21 local artists have been invited to share their visions for the city's creative future.
British architecture firm Foster + Partners have a longstanding arrangement with Apple, designing their flagship city stores that combine sleek minimalism with local design elements.
For the newly opened Macau store Foster + Partners filled the space with bamboo and pioneered a stone glass hybrid to create translucent walls that glow like a paper lantern, in contrast to the garish neon of casino-filled Cotai.
In Singapore, in order to reflect its status as the greenest city in Asia, the British architecture firm integrated sustainable technologies so that the shop is powered entirely by renewable energy.
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Main image courtesy of Apple, all other photos by Nigel Young.