"A serious accident took place around 5:40 am (0340 GMT). The bus deviated and fell into a ditch off the highway," Croatia's police spokesman Marko Muric said.
The death toll rose from 11 after one of the injured died in hospital, Interior Minister Davor Bozinovic told a press conference in the northern city of Varaždin.
Eighteen of the 30 hurt, who were all taken to hospitals in the area, were in a serious condition, said Maja Grba Bujevic, head of Croatia's emergency services.
Sixteen emergency medical teams had been dispatched to the crash site.
Top Polish officials, including the President and Prime Minister extended their deepest sympathies to the families of the victims.
PM Mateusz Morawiecki tweeted that the travelling Poles "were all pilgrims heading for Medjugorje," a Catholic sanctuary in neighbouring Bosnia that draws hundreds of thousands of visitors every year.
The group also included three priests and six nuns, according to the Polish Radio IAR news agency.
"All the victims (are) Polish citizens and the bus was registered in Poland" said Łukasz Jasina, a spokesperson for Poland's foreign ministry.
The people injured in the accident are in five hospitals, including three in the capital Zagreb, deputy Foreign Ministry Marcin Przydacz was quoted as saying by Poland's PAP news agency on Saturday. He added that some of the casualties were in a serious condition, some were able to communicate.
Przydacz was expected to arrive in Croatia later in the day, alongside health minister Adam Niedzielski.
Croatia has attracted millions of tourists this summer, drawn to its stunning Adriatic coast. About 580,000 Polish tourists visited Croatia between January and July this year.
In July, 10 tourists from Kosovo died in a bus crash on a highway in eastern Croatia.
Source: IAR, PAP, AFP