Bulgari, bought in 2011 by French luxury giant LVMH, is putting €1.5 million ($1.7 million) into the refurbishment.
The 18th-century Baroque-style stairway, which has 144 steps, was last restored 20 years ago.
The work, which will be carried out by local restorer Il Cenacolo, is expected to be finished by spring 2016, at which point the steps will be "restored to the whole world in all their beauty and splendour," Rome Mayor Ignazio Marino said.
The steps will be closed to pedestrians until December 7th - the start of a special Jubilee year expected to draw millions of Catholic pilgrims to Rome - after which a side ramp will be opened to allow access to tourists.
"Rome has always been the number one source of inspiration for Bulgari so it is right to give back to Rome what Rome has given Bulgari," the jeweller's CEO Jean-Christophe Babin said as work began.
The work includes re-levelling the steps to repair natural wear and maintenance of the rainwater drainage system, as well as restoring the original lamps which illuminate the steps by night.
The Spanish Steps, which links Bernini's "boat" fountain with the Trinita dei Monti church at the top, were made famous in the United States by the 1953 film Roman Holiday, starring Audrey Hepburn and Gregory Peck.
Bulgari pledged the money last year in response to a government plea for help in financing Italy's heritage monuments.
The private sector is already helping restore other Unesco monuments: in Rome, luxury shoemaker Tod's is financing works at the Colosseum, while high-end fashion house Fendi is refurbishing the Trevi Fountain.