The accident occurred during the night of Sunday on the M3 motorway, near the town of Mezőkeresztes.
The Polish-registered bus had 57 passengers and two drivers on board and was en route from Serbia to Poland, police said.
In addition to the 12 dead, at least 10 people are seriously injured, Hungarian Prime Minister Péter Magyar wrote in a statement on social media, in which he expressed his condolences to the victims' relatives.
The accident is the worst road accident in Hungary since 2003, when a German tourist bus collided with a train in Siófok and 33 people died, AFP writes.
