Admiral Sir William Reginald Hall KCMG CB (28 June 1870 – 22 October 1943), known as Blinker Hall, was the British Director of Naval Intelligence from 1914 to 1919. Together with Sir Alfred Ewing, he was responsible for the establishment of the Royal Navy's codebreaking operation, Room 40, which decoded the Zimmermann telegram, a major factor in the entry of the United States into World War I.

Reginald Hall was born at Britford, near Salisbury in Wiltshire, on 28 June 1870. He was the eldest son of Captain William Henry Hall, the first head of Naval Intelligence, who had married the daughter of the Reverend George Armfield from Armley, Leeds. Hall decided on a naval career for himself when taken on a cruise on board HMS Flamingo by his father.

Hall joined the training ship HMS Britannia in 1884 and two years later was appointed to the armoured cruiser Northampton. After a year he was transferred to the ironclad battleship Bellerophon which was part of the North American Station. In 1889 he became acting sub-lieutenant before attending courses for his Lieutenant's examinations at Greenwich, the gunnery school and torpedo school, where he achieved first class grades in all five of the subjects. Now as Lieutenant he was posted to the China Station where he served on the flagship, the armoured cruiser Imperieuse. In 1892 he was recommended to train as a gunnery officer, which involved a course in mathematics at the Royal Naval College, Greenwich, followed by a year's course at the gunnery school at HMS Excellent, a shore establishment at Whale Island, Portsmouth. Having qualified, he remained for a year on the staff.

In 1894 Hall married Ethel Wootton de Wiveslie Abney, daughter of Sir William. The engagement had begun when he was aged 19, five years before, but at the time this was considered a young age for a naval officer to get married. In 1895 he was appointed as gunnery lieutenant on the cruiser Australia where he served for two years before being appointed to the senior staff at Whale Island. He was promoted to commander 1 January 1901, in recognition of services with naval forces in South Africa, and later served on the battleship Magnificent, flagship of the second in command of the Channel Fleet.

In 1904 he became commander of the pre-dreadnought battleship Cornwallis which was commissioning with a new crew to operate in the Mediterranean. Hall had a reputation as a strict disciplinarian with an ability to get the best out of a crew, so he was given some of the worst sailors to bring up to scratch. A trick of disaffected sailors was to remove gunsights and throw them overboard, and indeed this happened. Hall instructed the master at arms to look out for two sailors who had previously been friends but who now avoided each other. He took one, told him that the other had confessed they had removed the sights, and the sailor duly confessed. Behaviour on board improved. Although Hall imposed discipline strictly, he also showed concern for the men he commanded and for their welfare, which was unusual for that time. He took particular care for the boys and junior seamen.

In December 1905 he was promoted to captain and appointed by the First Sea Lord, Sir John Fisher, as Inspecting Captain of the new Mechanical Training Establishments, which Fisher had established to give engineering training to ordinary sailors (1906–7). He was next appointed captain of the cadet training ship HMS Cornwall. Although not a conventional warship, this now involved Hall in intelligence work. The ship visited foreign ports, particularly in Germany which was now seen as the navy's greatest potential enemy, and Hall started the tour with a long list of places to investigate. In Kiel he was tasked with discovering how many slips had been constructed for building large vessels. To do this he devised a ruse with the help of the Duke of Westminster, who was visiting the port and agreed to lend his motor boat for the task. Hall and a couple of officers dressed down as sailors and took the yacht on a full speed circuit of the harbour, pretending to break down by the naval dockyard. A concealed camera was then used to take photographs of the installations. More information was discovered by careful questions ashore.

In 1910 two officers, Lieutenant Brandon and Captain Trench, who had been part of Hall's crew gathering intelligence, were sent on a 'holiday' in Germany to collect information about coastal defences by Captain Regnart of the intelligence division. The two were captured and served two-and-a-half years of a four-year sentence, before being pardoned in May 1913 as part of a visit by King George V to Germany. The admiralty then denied any responsibility for what had happened to the pair and their considerable financial loss as a result. When appointed director of intelligence, Hall arranged for them to be compensated.

Hall was appointed to the armoured cruiser Natal following the death of her captain, F.C.A. Ogilvy. Ogilvy had obtained a high reputation for his ship which had beaten all records at gunnery but Hall managed to step into Ogilvy's place, retaining the confidence of the crew and bettering the gunnery record the following year. His reputation for unorthodox treatment of his men continued to grow, making it his business to reform recalcitrant sailors rather than simply punishing them. He had the knack of threatening men with punishments he could not deliver (such as dismissal from the service) if they did not reform, and succeeded. Natal was called upon to assist a sailing ship, the Celtic Race, which had lost most of her sails in a storm and was in danger of sinking. Despite the risk to his own ship, he escorted Celtic Race into Milford Haven. He was rewarded by the owners and underwriters, being presented with a silver table centrepiece by the Lord Mayor of Liverpool on their behalf.