An independence referendum was held on 1 October 2017 in the Spanish autonomous community of Catalonia, passed by the Parliament of Catalonia as the Law on the Referendum on Self-determination of Catalonia and called by the Generalitat de Catalunya. The referendum, known in the Spanish media by the numeronym 1-O, was declared unconstitutional on 7 September 2017 and suspended by the Constitutional Court of Spain after a request from the Spanish government, who declared it a breach of the Spanish Constitution. Additionally, in early September the High Court of Justice of Catalonia had issued orders to the police to try to prevent the illegal referendum, including the detention of various persons responsible for its preparation. Due to alleged irregularities during the voting process, as well as the use of force by the National Police Corps and Civil Guard, international observers invited by the Generalitat declared that the referendum failed to meet the minimum international standards for elections.

Catalonia is an autonomous community of Spain, designated as a nationality by its Statute of Autonomy. Most of its territory lies on the northeast of the Iberian Peninsula, to the south of the Pyrenees mountain range. Catalonia is administratively divided into four provinces or eight regions, which are in turn divided into 42 comarques. The capital and largest city, Barcelona, is the second-most populated municipality in Spain and the fifth-most populous urban area in the European Union.