"We have done everything possible to save him but he never regained consciousness," doctor Leonardo Paloscia told reporters.
Another doctor at the Pescara hospital said he was dead on arrival at hospital.
The news comes just weeks after Bolton Wanderers midfielder Fabrice Muamba suffered the same problem in an English FA Cup match before making a remarkable recovery.
Morosini fell to the ground with no one around him in the 31st minute. He got up looking confused and then collapsed again as medics and then an ambulance rushed on to the field.
The Serie B game was abandoned with Livorno leading 2-0 and Morosini was taken to a Pescara hospital.
AC Milan were due to host Genoa in Serie A at 1600 GMT.
At first Milan said there would be a minute's silence but then the public announcer told fans already in the stadium that the game would be postponed along with all other remaining Serie A matches and the rest of the weekend's Serie B.
There was applause but also some whistles.
ANSA quoted Italian soccer federation director general Antonello Valentini as saying all professional games were off.
The Italian Olympic Committee (CONI), which confirmed Morosini had suffered a cardiac arrest, announced in a statement that all other sporting events this weekend would hold a minute's silence.
