William III (pictured) unilaterally revised the constitution of Luxembourg, greatly expanding his powers as grand duke.

William III was King of the Netherlands and Grand Duke of Luxembourg from 1849 until his death in 1890. He was also the Duke of Limburg from 1849 until the abolition of the duchy in 1866.

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.

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