John Meares (c. 1756 – 1809) was an Irish navigator, explorer, and maritime fur trader in service of Great Britain during the late 18th century. He is best known for his voyages to the Pacific Northwest; and from his base in Canton (modern day Guangzhou) he was deeply involved in organising a private network of British trade in China and the wider Pacific outside of the monopoly of the East India Company. His activities precipitated the Nootka Crisis, which brought Britain and Spain to the brink of war. He is also known for his early efforts to open up trade between feudal Japan and Great Britain.

Meares' father was Charles Meares, "formerly an attorney of great eminence, and for several years pursuivant of his Majesty's Court of Exchequer in Dublin". In 1771, Meares joined the Royal Navy as a captain's servant and was commissioned a lieutenant in 1778. In 1783 he joined the merchant service and in 1785, based in India, formed the Northwest America Company for collecting sea otter furs by trade with the indigenous peoples of the Pacific Northwest Coast and selling them in China. The East India Company held a monopoly on British trade in the Pacific and required all British traders to be licensed with the company and pay duties. Meares did not license his ships with the East India Company and instead tried to conceal the illegal activity by using the flag of Portugal. Meares registered his ships in Macau, a Portuguese colony in China. Non-British ships were not required to have licences from the East India Company.

He sailed from Calcutta on 12 March 1786, in the Nootka, a vessel of 200 tons (bm), with which he explored part of the coast of Alaska. He spent the winter of 1786–1787 in Prince William Sound with poor provisions; his men suffered from the weather and scurvy. Twenty-three of his men died of scurvy and the remaining ten were saved only by the timely arrival of Captain George Dixon, a British trader with proper licences, in the Queen Charlotte. Meares gave Dixon his bond never to trade in the Northwest again, and returned to China by way of the Sandwich Islands. Showing no gratitude to Dixon, Meares proceeded to sue him for allegedly overcharging him for the supplies that saved his life.

In 1788, and in total violation of what he had told Dixon, Meares started a new expedition with two vessels and more false papers. The ships sailed under the Portuguese flag and were given Portuguese names: the Felice Adventurero, captained by Meares, and the Iphigenia Nubiana, under William Douglas. The names are spelled in various ways, such as Feliz Aventureira and Efigenia Nubiana. They sailed from China on 22 January 1788 and arrived at Nootka Sound, on Vancouver Island in May. Using Nootka Sound as a base of operations he spent the summer trading for furs along the coasts of present-day British Columbia and Washington down to after having sailed as far south as Cape Lookout, having missed the Columbia River at Cape Disappointment.

On board Felice Adventurero for the 1788 voyage from China to Nootka Sound was Comekela, younger brother of Maquinna, chief of the Mowachaht Nuu-chah-nulth people of the Nootka Sound area. Comekela had sailed from Nootka Sound to China in 1786-87 with James Hanna.

Meares later claimed that Maquinna, a chief of the Nuu-chah-nulth (Nootka) people, sold him some land on the shore of Friendly Cove in Nootka Sound, in exchange for some pistols and trade goods, and that on this land some kind of building was erected. These claims would become a key point in Britain's position during the Nootka Crisis. Spain strongly disputed both claims, and the true facts of the matter have never been fully established. Maquinna himself later called Meares a liar and denied selling him any land. The land and building aside, there is no doubt that Meares' men, and a group of Chinese workers they brought, built the sloop North West America. It was launched in September 1788, the first non-Indigenous vessel built in the Pacific Northwest.

In September Meares left, sailing the Felice Adventurero to China. The Iphigenia Nubiana and North West America wintered in the Hawaiian Islands. While on the island of Kauai, Meares picked up a Hawaiian chief or "prince of Attooi" (Kauai) by the name of Tianna (Kaʻiana – also spelled Tyaana & Tyanna), whom he took aboard his ship. He took Tianna to Guangzhou (Canton), China, where Meares found a Hawaiian woman by the name of Wynee, who had been left there by captain Charles William Barkley of the Imperial Eagle. Eventually, Meares returned Tianna to his homeland but Wynee died of illness on the voyage home. Wynee and Tianna are considered the first two Hawaiians to have traveled abroad. Later, Meares gained possession of Barkley's nautical gear and his journal. Barkley's wife, Frances Barkley, later wrote that Meares, "with the greatest effrontery, published and claimed the merit of my husband's discoveries therein contained, besides inventing lies of the most revolting nature tending to vilify the person he thus pilfered."

During the winter of 1788–89 Meares was in Guangzhou (Canton), China, where he and others formed a partnership called the Associated Merchants Trading to the Northwest Coast of America. Plans were made for more ships to sail to the Pacific Northwest in 1789, including the Princess Royal, under Thomas Hudson, and the Argonaut under James Colnett. The consolidation of the fur trading companies of Meares and the Etches (King George's Sound Company) resulted in James Colnett being given the overall command. Meares instructed Colnett to establish a permanent fur trading post at Nootka Sound based on the foothold accomplished by Meares the year before. The Iphigenia Nubiana and North West America were to join the Argonaut and Princess Royal at Nootka Sound. Meares himself remained in China.