Adelaide of Saxe-Meiningen (Adelaide Amelia Louise Theresa Caroline; 13 August 1792 – 2 December 1849) was Queen of the United Kingdom of Great Britain and Ireland and Queen of Hanover from 26 June 1830 to 20 June 1837 as the wife of King William IV. Adelaide was the daughter of George I, Duke of Saxe-Meiningen, and Luise Eleonore of Hohenlohe-Langenburg. Adelaide, the capital city of South Australia, is named after her.

Adelaide was born on 13 August 1792 in Meiningen, Thuringia, Germany, the eldest child of Georg I, Duke of Saxe-Meiningen, and Luise Eleonore, daughter of Christian Albrecht, Prince of Hohenlohe-Langenburg. She was baptised at the castle chapel on 19 August and was titled Princess Adelaide of Saxe-Meiningen, Duchess in Saxony with the style Serene Highness. Her godparents numbered 21, including her mother, the Holy Roman Empress, the Queen of Naples and Sicily, the Crown Princess of Saxony, the Duchess of Saxe-Gotha-Altenburg, the Duchess of Saxe-Coburg, the Duchess of Saxe-Weimar, the Prince of Hohenlohe-Langenburg, and the Landgrave of Hesse-Philippsthal-Barchfeld.

Saxe-Meiningen was a small state, covering about 423 square miles (1,100 km²). It was the most liberal German state and, unlike its neighbours, permitted a free press and criticism of the ruler. At the time no statute existed barring a female ruling over the small duchy and it was not until the birth of her brother, Bernhard, in 1800 that the law of primogeniture was introduced.

By the end of 1811, King George III was incapacitated and, although he was still king in name, his heir-apparent and eldest son, Prince George, was regent. On 6 November 1817, the Prince Regent's only legitimate child, Princess Charlotte, died in childbirth. Princess Charlotte was second in line to the throne: had she outlived her father and grandfather, she would have become queen. With her death, the King was left with twelve children and no legitimate grandchildren. The Prince Regent was estranged from his wife, who was 49 years old, thus there was little likelihood that he would have any further legitimate children. To secure the line of succession, Prince William, Duke of Clarence and St Andrews, and the other sons of George III sought quick marriages with the intent of producing offspring who could inherit the throne. William already had ten children by the popular actress Dorothea Jordan, but, being illegitimate, they were barred from the succession.

Considerable allowances were likely to be voted by Parliament to any royal duke who married, and this acted as a further incentive for William to marry. While Adelaide was a princess from an unimportant German state, William had a limited choice of available princesses, and, after deals with other candidates fell through, a marriage to Adelaide was arranged. The allowance proposed was slashed by Parliament, and the outraged Duke considered calling off the marriage. However, Adelaide still seemed the ideal candidate of a wife: amiable, home-loving, and willing to accept William's illegitimate children as part of the family. The arrangement was settled and William wrote to his eldest son, "She is doomed, poor dear innocent young creature, to be my wife." William eventually accepted the reduced increase in his allowance voted by Parliament, her dowry was set at 20,000 florins, with three additional separate annuities being promised by her future husband, the Prince Regent, and the state of Saxe-Meiningen.

Adelaide married William in a double wedding with William's brother, Prince Edward Augustus, Duke of Kent and Strathearn, and his bride Victoria, Dowager Princess of Leiningen, on 11 July 1818, at Kew Palace in Surrey, England. They had met for the first time only a week earlier on 4 July at Grillon's Hotel in Albemarle Street. Neither William nor Adelaide had been married before, and William was 27 years her senior. Despite these unromantic circumstances, the couple settled amicably in Hanover, where the cost of living was much lower than in England, and by all accounts were devoted to each other throughout their marriage. Adelaide improved William's behaviour; he drank less, swore less, and became more tactful. Observers thought them parsimonious, and their lifestyle simple.

Adelaide soon became pregnant but in her seventh month of pregnancy she developed pleurisy and gave birth prematurely on 27 March 1819 at the Fürstenhof Palace in Hanover. Her daughter, Charlotte Augusta Louise, lived only a few hours. Another pregnancy in the same year caused William to move the household to England so his heir would be born on British soil; however Adelaide miscarried at Calais or Dunkirk during the journey on 5 September 1819. Back in London they moved into Clarence House but preferred to stay at Bushy House near Hampton Court, where William had already lived with Dorothea Jordan. Adelaide became pregnant again, and a second daughter, Elizabeth Georgiana Adelaide, was born on 10 December 1820 at St James's Palace. Elizabeth seemed strong but died at less than three months old on 4 March 1821 of "inflammation in the Bowels". Twin boys were stillborn on 8 April 1822 at Bushy Park and a possible brief pregnancy may have occurred in the same year. Ultimately William and Adelaide had no surviving children.

At the time of their marriage, William was not heir-presumptive to the throne, but became so when his brother Frederick, Duke of York, died childless in 1827. Given the small likelihood of his older brothers producing heirs, and William's relative youth and good health, it had long been considered extremely likely that he would become king in due course. In 1830, on the death of his elder brother, George IV, William acceded to the throne. He and Adelaide were crowned on 8 September 1831 at Westminster Abbey. William despised the ceremony and acted throughout, it is presumed deliberately, as if he was "a character in a comic opera", making a mockery of what he thought to be a ridiculous charade. In contrast, Adelaide took the service very seriously and, among those attending, received praise for her "dignity, repose and characteristic grace".