Alexander Dalrymple FRS (24 July 1737 – 19 June 1808) was a Scottish geographer, hydrographer, and publisher. He spent the greater part of his career with the British East India Company, starting as a writer in Madras at the age of 16. He studied the old records of the company, and soon became sufficiently knowledgeable to advise on shipping routes in the East Indies. He spent several years travelling, investigating possibilities of expanding the company's trade, and carried out extensive surveys around Borneo, the Philippines, and Indo-China. Returning to England, he published a range of works including charts, histories of past voyages, and proposals for exploration. He was one of the main proponents of the theory that there existed a great undiscovered continent in the South Pacific, Terra Australis Incognita. He was The Royal Society's first choice as leader of the exploration to observe the transit of Venus in 1769, a position taken by James Cook as the Navy would not accept a non-naval man in command. A large part of Cook's first two voyages was in search of the conjectured southern land, leading to the conclusion that, if it did exist, it was further south than the 65° line of latitude. He became Hydrographer to the East India Company, and then the first Hydrographer of the British Admiralty. He produced large numbers of nautical charts and sailing directions many of which remained in print long after his death, contributing significantly to the safety of shipping.

Dalrymple was born at Newhailes, near Edinburgh, the eleventh of fifteen children of Sir James Dalrymple and his wife, Lady Christian Hamilton, the daughter of the Earl of Haddington. His father served as the Principal Auditor of the Exchequer in Scotland from 1721 until his death, and was the Member of Parliament for Haddington Burghs from 1722 to 1734. Alexander attended David Young's School in Haddington. The Dalrymples were opposed to the Stuart dynasty, and the family moved to Berwick-on-Tweed for safety during the 1745-6 Jacobite uprising, after which Alexander returned to school until he was fourteen. His father died in 1751, and with the help of his uncle General St. Clair, who knew the chairman of the British East India Company, an appointment was arranged for Alexander as a writer in the company. He travelled to London in 1752, then sailed to Madras in the Suffolk East-Indiaman, commanded by Captain William Wilson, who befriended the young man on the voyage. He arrived in Madras, aged 16, in 1753.

Dalrymple's uncle had recommended him to Lord Pigot, who became Governor of Madras in 1755. He also became acquainted with the historian Robert Orme who allowed him access to his library. Dalrymple took the opportunity to study the old records of the company, and soon became knowledgeable about previous voyages in the East Indies. This proved its worth in 1758 when Captain Wilson arrived back in Madras in command of the Pitt East-Indiaman. Wilson was intending to continue to Canton, in China, but had been delayed, and it was too late in the season to sail by the standard route through the South China Sea as the contrary north-east Monsoon would start in October. The normal procedure would have been to wait several months for the return of the south-west monsoon, but Wilson was thinking of a different route, heading south-east to Batavia, now Jakarta, then east and north-east to the north-west coast of New Guinea, then north-west to Canton, with the monsoon winds on his beam. This route was longer, and parts of it were not well known to British sailors, but the winds were favourable. Wilson discussed this with Dalrymple, who was able to confirm its practicality on the basis of voyages in the previous century. Wilson took this route, and successfully completed his voyage six months earlier than expected. Off the north-west coast of New Guinea, Wilson passed through a channel he named the Pitt Strait.

The voyage was important for the company, as the route became a standard alternative that opened up China navigation at any time of year.

While in Madras, Dalrymple had become interested in new possibilities of trade in the East Indies. He realised that Wilson's route took him close to the Sulu Sea, where there were numerous islands that were not controlled by rival Dutch or Spanish interests, and that could form a good trading base for the company. Dalrymple proposed a voyage of discovery to the Sulu Archipelago to Lord Pigot, the governor, and also expressed his wish to lead it in person. Pigot tried to dissuade him from this, as his absence on a long voyage would likely impair his promotion prospects, but Dalrymple was determined, and Pigot agreed.

Preparations were delayed by the Siege of Madras (December 1758 – February 1759) by French forces, part of the Seven Years' War. Madras was subjected to heavy bombardment, but was able to hold out until reinforcements arrived. One of the casualties of the bombardment was a supercargo called William Roberts. After his death his assets were sold, and Dalrymple was able to buy some Spanish books on the history and geography of the Philippines, which were very useful in planning his expedition.

The vessel chosen for the voyage was the snow Cuddalore, which sailed to Malacca under the command of George Baker, while Dalrymple travelled as a passenger on the Winchelsea with Thomas Howe, an experienced captain who took the opportunity to teach Dalrymple navigation and seamanship. In Malacca, Dalrymple transferred to Cuddalore, initially under the joint command of Baker and himself. They sailed to Macao, and had to wait for supplies to arrive from Europe. Dalrymple decided to explore and survey the islands to the north of the Philippines which were on the Wilson route to China. They left Macao on 27 July 1759, returning on 13 October. Baker resigned his command on 24 November, leaving Dalrymple in sole command.

There were then difficulties with the authorities in Macao over the return of some seamen who had deserted earlier, leading to a delay of several months. The Cuddalore then made a brief visit to Touranne (Da Nang) and Cape Padarin in Cochin China, now Vietnam. This took him to regions little visited by the British, including the coast of Hainan, and Dalrymple sent his observations directly to William Pitt, Secretary of State, thinking they might be useful in case of hostilities with the Chinese. While relations with China at this stage were difficult, the active hostilities were with France, whose forces had taken much of Sumatra and were blockading the Sunda Strait, the normal route for East-Indiamen sailing from China to England. Dalrymple was asked by the Committee of Supercargoes in Canton to assist a convoy of five ships on an alternative route by way of the Sulu Sea, the Makassar Strait, and then through the Sape Strait into the Indian Ocean, after which sailing to England was straightforward. Much of this route was uncharted, and the Cuddalore could act as scout for the larger vessels. Dalrymple readiy agreed, as the route would take them to the Sulu archipelago, the main target of his plans.