A constitutional crisis took place in Spain from 2017 to 2018 as the result of a political conflict between the Government of Spain and the Generalitat de Catalunya under the then-President Carles Puigdemont—the government of the autonomous community of Catalonia until 28 October 2017—over the issue of Catalan independence. It started after the law intending to allow the 2017 Catalan independence referendum was denounced by the Spanish government under Prime Minister Mariano Rajoy and subsequently suspended by the Constitutional Court until it ruled on the issue. Some international media outlets have described the events as "one of the worst political crises in modern Spanish history".

In political science, a constitutional crisis is a problem or conflict in the function of a government that the political constitution or other fundamental governing law is perceived to be unable to resolve. There are several variations to this definition. For instance, one describes it as the crisis that arises out of the failure, or at least a strong risk of failure, of a constitution to perform its central functions. The crisis may arise from a variety of possible causes. For example, a government may want to pass a law contrary to its constitution; the constitution may fail to provide a clear answer for a specific situation; the constitution may be clear but it may be politically infeasible to follow it; the government institutions themselves may falter or fail to live up to what the law prescribes them to be; or officials in the government may justify avoiding dealing with a serious problem based on narrow interpretations of the law. Specific examples include the South African Coloured vote constitutional crisis in the 1950s, the secession of the southern U.S. states in 1860 and 1861, the dismissal of the Australian federal government in 1975 and the 2007 Ukrainian crisis. While the United Kingdom of Great Britain and Northern Ireland does not have a codified constitution, it is deemed to have an uncodified one, and issues and crises in the UK and its constituent countries are described as constitutional crises.