"We have done everything possible to save him but he never regained consciousness," doctor Leonardo Paloscia said.
Another doctor at the Pescara hospital said the 25-year-old, a former Italy under-21 international who was at Udinese until January, was dead on arrival at hospital.
The news comes just weeks after Bolton Wanderers midfielder Fabrice Muamba suffered a similar problem in an FA Cup match before making a remarkable recovery.
Serie A said in a statement that the federation had postponed all the weekend's professional matches.
Morosini, born in Bergamo and a former player with the city's Atalanta team, 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, who was orphaned as a teenager and also lost a brother, was taken to a Pescara hospital where he was pronounced dead.
Defending Serie A champions Milan were due to host Genoa in Serie A that afternoon.
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 this weekend.
There was applause but also some whistles.
The Italian Olympic Committee (CONI), which confirmed Morosini had suffered a cardiac arrest, announced in a statement that all other sporting events taking place this weekend would hold a minute's silence.
The issue of players' health has long been in focus across the game with extra medical and heart checks being brought in throughout many leagues.
Marc-Vivien Foe collapsed during Cameroon's 1-0 victory against Colombia in a Confederations Cup match in June 2003 in Lyon, France and died shortly afterwards aged 26.
The shock over that high-profile death, one of at least 20 in the last 20 years in professional leagues, led to changes but some believe more needs to be done.
In the wake of Bolton's Muamba suffering a cardiac arrest while playing at Tottenham Hotspur in an FA Cup quarter-final on March 17, Manchester City manager Roberto Mancini - an Italian - said English medicals were not as good as in his homeland.
Muamba too fell to the ground with no one near him and doctors later said he had been "in effect dead" for 78 minutes while his heart did not beat on its own. He was revived and has gone on to make extraordinary progress in hospital.
He is now walking and recently tweeted a photograph of himself sitting up in bed and smiling.
The Italian medical system hailed by Mancini did not help Morosini, however. Tearful Livorno players travelled to the hospital to pray alongside his body.
Italy is used to sporting tragedy in recent years.
Last October, popular MotoGP rider Marco Simoncelli died after a crash at the Malaysian Grand Prix.
Franco Ballerini, the coach of Italy's cycling team, also died in a rallying accident in 2010.