The attack took place at dawn in the town of Salvatierra when an armed group opened fire on partygoers attending a posada, a traditional Mexican party held in the days leading up to Christmas, according to local media.
The state's attorney general office said on X that 12 people had been killed without giving any more details.
Guanajuato, home to a significant number of automotive and aeronautics factories, has in recent years become one of the most violent states in the country, amid clashes between powerful drug trafficking groups.