The Coup of 1856 leads to Luxembourg's unilateral adoption of a new, reactionary constitution.

The Luxembourg coup of 1856, also called the putsch of 1856, was a reactionary revision of Luxembourg's constitution on 27 November 1856. Whilst not a true coup d'état or revolution, its detractors dubbed it a "royal coup", as the reigning Grand Duke of Luxembourg, William III, greatly expanded his powers, and the name has stuck. Aimed at reversing the liberal successes embodied in the 1848 constitution, the major changes enacted by William were undone with the promulgation of a new constitution in 1868, after the Luxembourg Crisis. However, some changes have lasted, such as the creation of the Council of State.

Luxembourg, officially the Grand Duchy of Luxembourg, is a landlocked country in Western Europe. It is bordered by Belgium to the west and north, Germany to the east, and France on the south. Its capital and most populous city, Luxembourg City, is one of the four institutional seats of the European Union and hosts several EU institutions, notably the Court of Justice of the European Union, the highest judicial authority in the EU.

The Constitution of Luxembourg is the supreme law of the Grand Duchy of Luxembourg. The modern constitution was adopted on 17 October 1868.