Ernest Augustus, Crown Prince of Hanover (German: Ernst August; 21 September 1845 – 14 November 1923), was the eldest child and only son of George V of Hanover and his wife, Marie of Saxe-Altenburg. Ernest Augustus was deprived of the throne of Hanover upon its annexation by Prussia in 1866 and later the Duchy of Brunswick in 1884. He was also deprived of his British peerages (including Duke of Cumberland and Teviotdale) and honours for having sided with Germany in World War I.

Prince Ernest Augustus of Hanover, Duke of Brunswick-Lüneburg, Prince of Great Britain and Ireland, was born at Hanover during the reign of his paternal grandfather, Ernest Augustus, King of Hanover. He became the crown prince of Hanover upon his father's accession as George V in November 1851.

In 1866, William I of Prussia and his minister-president Otto von Bismarck deposed George V and annexed Hanover after George V sided with the defeated Austria in the 1866 Austro-Prussian War. During that war, the Crown Prince saw action at the Battle of Langensalza.

After the war, the exiled Hanoverian royal family took up residence in Hietzing, near Vienna, but spent a good deal of time in Paris. George V never abandoned his claim to the Hanoverian throne and maintained the Guelphic Legion at his own expense. The former Crown Prince travelled during this early period of exile, and ultimately accepted a commission in the Imperial and Royal Army of Austria-Hungary. The Guelph Party, or the German-Hanoverian Party, as a minor party in the legislature of the North German Federation and then the German Empire continued to protest the annexation of Hanover and advocated for the restoration of the state of Hanover with a Guelph at its head.

When King George V died in Paris on 12 June 1878, Prince Ernest Augustus succeeded him as Duke of Cumberland and Teviotdale in the Peerage of Great Britain and Earl of Armagh in the Peerage of Ireland. Queen Victoria created him a Knight of the Garter on 20 July 1878. Emperor Franz Joseph I of Austria appointed him to succeed his father as colonel and proprietor of the Austrian 42nd Regiment of Infantry. The regiment's name was changed to honour him, and he served as its honorary colonel from 1879 to the fall of the Austro-Hungarian Empire, in 1918.

While visiting his second cousin Albert Edward, Prince of Wales (later King Edward VII) at Sandringham in 1875, he met Princess Thyra of Denmark (29 September 1853 – 26 February 1933), the youngest daughter of King Christian IX and a sister of the Princess of Wales (later Queen Alexandra).

On 21/22 December 1878, he and Princess Thyra married at Christiansborg Palace in Copenhagen.

Queen Victoria appointed the Duke of Cumberland a colonel in the British Army in 1876 and promoted him to major general in 1886, lieutenant general in 1892 and general in 1898. Although he was a British peer and a prince of Great Britain and Ireland, he continued to consider himself an exiled monarch of a German realm and refused to disclaim his succession rights to Hanover, making his home in Gmunden, Upper Austria.