A constitutional referendum was held in Spain on Wednesday, 6 December 1978, to gauge support for either the ratification or repealing of the Spanish Constitution which had been approved by the Cortes Generales on 31 October 1978. The question asked was "Do you approve of the Constitution Bill?" (Spanish: ¿Aprueba el Proyecto de Constitución?). The referendum resulted in 91.8% of valid votes in support of the bill on a turnout of 67.1%.
Some Spanish media found up to 30% of irregularities in the census in certain provinces, with many people allegedly being unable to vote while others voted twice.Adolfo Suárez's government had lowered voting age from 21 to 18 only three weeks before the referendum, which resulted in a made-up electoral register increasing by over 3 million people compared to the 1977 general election amid technical, administrative and logistical issues.Interior Ministry officials acknowledged deviations of up to 5.1 per 100 in the electoral census—roughly 1.5 million people according to the National Institute of Statistics—resulting from the absence of an official electoral register and in an overreliance on data from municipal registers.