William (German: Wilhelm August Ludwig Maximilian Friedrich; born 25 April 1806 in Brunswick, Brunswick-Wolfenbüttel – died 18 October 1884 in Sibyllenort, Silesia, Prussia), Duke of Brunswick, was ruling duke of the Duchy of Brunswick from 1830 until his death.

William was the second son of Frederick William, Duke of Brunswick-Lüneburg, and after the death of his father in 1815, was under the guardianship of King George IV of the United Kingdom. He became a Prussian major in 1823. When his brother, Charles, was deposed as ruling duke by a rebellion in 1830, William took over government provisionally. In 1831 a family law of the House of Guelph made William ruling duke permanently. William left most government business to his ministers, and spent most of his time outside of his state at his possessions in Oels.

While William joined the Prussian-led North German Confederation in 1866, his relationship to Prussia was strained, since Prussia refused to recognize Ernest Augustus, 3rd Duke of Cumberland, his nearest male-line relative, as his heir, because of the Duke of Cumberland's claim to the throne of Hanover. William died in 1884; he passed on his private possessions to the Duke of Cumberland. His death caused a constitutional crisis for Brunswick that lasted until the accession of Ernest Augustus, Duke of Brunswick, the son of the Duke of Cumberland, in 1913.

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