Its official website (www.worldsbk.com) said the 35-year-old, who rides for the Red Bull Honda team, was hit while training near the east coast resort of Rimini following a race at Imola.
"Ambulances arrived and Hayden was taken directly to the hospital," it added.
The statement did not say how severe any injuries might be. Media reports said the "Kentucky Kid", who won the MotoGP championship on a Honda in 2006, was in a serious condition.
He last raced in the championship in Spain in September 2016 as a stand-in for injured Australian Jack Miller at the privately-run Marc VDS Honda team. Hayden started 216 races between 2003 and 2015, winning three.
Michele Scarponi, the 2011 Giro d'Italia cycling champion, was killed last month after being hit by a van while training close to his home in Filottrano in eastern Italy.
Britain's Tour de France champion Chris Froome escaped unhurt this month while having his bike written off in a hit-and-run incident in France.
(Reporting by Alan Baldwin; editing by Mark Heinrich)
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