More than 100 new LED lamps were installed to better illuminate the details of the fountain's centrepiece, while the plumbing was replaced to ensure the water's pH level will be maintained to protect the marble.
The nearly 300-year-old monument, visited by millions of tourists every year, hit the headlines in 2012 after bits of its elaborate cornice began falling off following a particularly harsh winter.
Now the fountain, made famous by a scene in Federico Fellini's "La Dolce Vita" in which Anita Ekberg wades through its pristine waters, makes the surrounding buildings in Rome's historic centre look decidedly shabby.
As the water cascaded once more from the base of the fountain's sea-god scene, a hail of coins plopped into the refilled basin as the Italian house's designer Silvia Venturini Fendi looked on.
The basin was drained but the most determined visitors had still been slinging coins over their shoulders -- a tradition that is said to ensure a return to Rome -- in the hope of getting them into a small substitute pool.
Frustrated tourists have spent months peeking at bits of the monument from a special walkway put in over the fountain while repairs, funded by Italian fashion house Fendi, were carried out to the tune of over two million euros ($2.2 million).
Crowds packed the small square in the Eternal City's historic centre for the ceremony, with excited couples hugging and kissing next to the ornate fountain where tradition has it they can ensure eternal love by drinking its waters.
"It's very emotional, especially for us Romans," Fendi said, after the company's CEO Pietro Beccari announced that the fashion house would fund the restoration of four other fountains in the Italian capital in a "gesture of love" for Rome.