Romboni, 45, was flown by helicopter to hospital in Latina, south of Rome, after the crash in practice for the Supermoto event attended by other well known riders, including former world champion Max Biaggi.
The official motogp.com website said he had died from his injuries later in the afternoon.
Romboni competed between 1996-97 as an Aprilia factory rider in the 500cc category that is now MotoGP and was a six times race winner in the 125cc and 250cc categories. He also raced in world superbikes.
"Grew up watching him when I was a kid, such a great rider," said American former world superbike champion and MotoGP race winner Ben Spies on Twitter. "Prayers to his family and friends."
Other riders, including Italy's Loris Capirossi and Marco Melandri, sent their condolences.
Biaggi said those taking part in the event had decided to continue on Sunday to support Romboni's family. The dead rider had three children.
(Writing by Alan Baldwin in London, editing by Justin Palmer and Ken Ferris)
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