Ernest Augustus (German: Ernst August; 5 June 1771 – 18 November 1851) was King of Hanover from 20 June 1837 until his death in 1851. As the fifth son of George III of the United Kingdom and Hanover, he initially seemed unlikely to become a monarch, but none of his older brothers had a legitimate son. When his brother William IV, who ruled both kingdoms, died in 1837, his niece Victoria inherited the British throne under British succession law, while Ernest succeeded in Hanover under Salic law, which barred women from the succession. This ended the personal union between Britain and Hanover that had begun in 1714. He remained heir presumptive to the British throne until the birth of Victoria, Princess Royal in 1840.

Ernest was born in London but was sent to Hanover in his adolescence for his education and military training. While serving with Hanoverian forces near Tournai against Revolutionary France, he received a disfiguring facial wound. He was created Duke of Cumberland and Teviotdale in 1799. Although his mother, Queen Charlotte, disapproved of his marriage in 1815 to her twice-widowed niece, Frederica of Mecklenburg-Strelitz, it proved happy. The eldest son of George III, the Prince of Wales (later George IV), had one child, Charlotte, who was expected to become the British queen, but she died in 1817, giving Ernest some prospect of succeeding to the British and Hanoverian thrones. However, his elder brother Prince Edward, Duke of Kent and Strathearn, fathered the eventual British heir, Victoria, in 1819 shortly before the birth of Ernest's only child, George.

Ernest was an active member of the House of Lords, where he maintained an extremely conservative record. There were persistent allegations (reportedly spread by his political foes) that he had murdered his valet, had fathered a son by his sister Sophia, and intended to take the British throne by murdering Victoria. Following the death of William IV, Ernest became Hanover's first resident ruler since George I. He had a generally successful fourteen-year reign but excited controversy near its start when he voided the liberal constitution granted before his reign and dismissed the Göttingen Seven, including the Brothers Grimm, from their professorial positions for protesting. In 1848, the King put down an attempted revolution. Hanover joined the German customs union in 1850 despite Ernest's reluctance. Ernest died the next year and was succeeded by his son, George V.

Ernest, the fifth son of King George III and Queen Charlotte, was born at Buckingham House, London, on 5 June 1771, and baptised on 1 July 1771 at St James's Palace. His sponsors were Duke Ernest of Mecklenburg (his maternal uncle), Moritz of Saxe-Gotha (his paternal great-uncle, for whom the Earl of Hertford stood proxy), and the Hereditary Princess of Hesse-Kassel (his father's cousin, for whom the Countess of Egremont stood proxy). After leaving the nursery, he lived with his two younger brothers, Prince Adolphus (later Duke of Cambridge) and Prince Augustus (later Duke of Sussex), and a tutor in a house on Kew Green, near his parents' residence at Kew Palace.

Though the King never left England in his life, he sent his younger sons to Germany in their adolescence. According to the historian John Van der Kiste, this was done to limit the influence Ernest's eldest brother George, Prince of Wales, who was leading an extravagant lifestyle, would have over his younger brothers. At the age of fifteen, Ernest and his two younger brothers were sent to the University of Göttingen, located in his father's Electorate of Hanover. Ernest proved a keen student and after being tutored privately for a year, while learning German, he attended lectures at the university. Though King George ordered that the princes' household be run along military lines and that they follow the university's rules, the merchants of the electorate proved willing to extend credit to the princes and all three fell into debt.

In 1790, Ernest asked his father for permission to train with the Prussian Army. Instead, in January 1791, he and Prince Adolphus were sent to Hanover to receive military training under the supervision of Field Marshal Wilhelm von Freytag. Before leaving Göttingen, Ernest penned a formal letter of thanks to the university and wrote to his father, "I should be one of the most ungrateful of men if ever I was forgetful of all I owe to Göttingen & its professors." Commissioned into the Hanoverian Army at the rank of lieutenant, Ernest learned cavalry drill and tactics under Captain Karl von Linsingen of the 9th (Queen's) Light Dragoons and proved to be an excellent horseman, as well as a good shot. After only two months of training, Freytag was so impressed by the Prince's progress that he appointed him as a captain in the regiment. Ernest was supposed to receive infantry training, but the King, also impressed by his son's prowess, allowed him to remain as a cavalryman.

In March 1792, the King commissioned Ernest as a colonel in the 9th (Queen's) Light Dragoons. The Prince served in the Low Countries during the War of the First Coalition under his elder brother Prince Frederick, Duke of York and Albany, the supreme commander of a Coalition army of British, Hanoverian and Austrian troops. In a skirmish with the French army near the Wallonian town of Tournai in August 1793, he sustained a sabre wound to the head, which resulted in a disfiguring scar. At the Battle of Tourcoing on 18 May 1794, his left arm was injured by a French cannonball passing close by him. In the days after the battle, the sight in his left eye faded. In June, he was sent to England to convalesce, his first stay there since 1786.

Ernest resumed his duties in early November, by now promoted to major general. He hoped his new rank would bring him a corps or brigade command, but none was forthcoming as Coalition troops retreated slowly through the Dutch Republic towards Germany. By February 1795, they had reached Hanover. Ernest remained in Hanover over the next year, holding several unimportant postings. He had requested a return home to seek treatment for his eye, but it was not until early 1796 that the King agreed and allowed Ernest to return to England. There, Ernest consulted eye surgeon Wathen Waller, but Waller apparently found his condition inoperable, as no operation took place. In England, Ernest repeatedly sought to be allowed to join Coalition forces on the Continent, even threatening to join the Yeomanry Cavalry as a private, but both the King and the Duke of York refused him permission. Ernest did not want to rejoin the Hanoverian Army, as they were not then involved in the fighting. In addition, Freytag was seriously ill and Ernest was unwilling to serve under his likely successor, Johann Ludwig, Reichsgraf von Wallmoden-Gimborn.