The Spanish Constitution of 1812 was promulgated by the Cortes Generales ("General Courts"), the national legislative assembly of Spain. The Cortes were taking refuge at Cadiz from the Peninsular War, called by the Spanish the Guerra de la Independencia which began on the night of May 2, immortalized by Goya in his paintings. The war was being fought on Spanish territory between Napoleon's forces and the Spanish partisans and the Allies under the Duke of Wellington.

The opening session of the new Cortes was held on September 24, 1810. Several basic principles were soon ratified: that sovereignty resides in the nation, the legitimacy of Ferdinand VII as King of Spain, and the inviolability of the deputies. The Cortes of Cadiz worked feverishly, and the first written Spanish constitution was promulgated in the city of Cadiz on March 12, 1812. Spain had been ruled as an absolute monarchy by the Bourbons and their Habsburg predecessors.

When Ferdinand VII was restored in March 1814 by the Allied Powers, he promissed to uphold the new charter of Spanish government, but within a matter of weeks, encouraged by conservatives backed by the Roman Catholic Church hierarchy, he repudiated the constitution (May 4) and arrested the liberal leaders (May 10), justifying his actions as repudiating a constitution made by the cortes in his absence and without his consent. Thus he had come back to assert the Bourbon doctrine that the sovereign authority resided in his person only.

When Ferdinand's ferocious misrule resulted in a mutiny of army officers in 1820, the Constitution of 1812 was the unifying document of the liberals, who wished to see a constitutional monarchy in Spain. After the Battle of Trocadero liberated Ferdinand in 1823, he turned on the liberals and constitutionalists with fury.

Since 1812, Spain has had a total of seven constitutions, including the one of 1978, currently in force as of 2004.