Jedediah Strong Smith (January 6, 1799 – May 27, 1831) was an American clerk, transcontinental pioneer, frontiersman, hunter, trapper, author, cartographer, mountain man and explorer of the Rocky Mountains, the Western United States, and the Southwest during the early 19th century. After 75 years of obscurity following his death, Smith was rediscovered as the American whose explorations led to the use of the 20-mile (32 km)-wide South Pass as the dominant route across the Continental Divide for pioneers on the Oregon Trail.

Coming from a modest family background, Smith traveled to St. Louis and joined William H. Ashley and Andrew Henry's fur trading company in 1822. Smith led the first documented exploration from the Salt Lake frontier to the Colorado River. From there, Smith's party became the first United States citizens to cross the Mojave Desert into what is now the state of California but which at that time was part of Mexico. On the return journey, Smith and his companions were likewise the first U.S. citizens to explore and cross the Sierra Nevada and the treacherous Great Basin Desert. The following year, Smith and companions were the first U.S. explorers to travel north from California overland to the Oregon Country. Surviving three Native American massacres and one bear mauling, Smith's explorations and documented travels were important resources to later American westward expansion.

In March 1831, while in St. Louis, Smith requested of Secretary of War John H. Eaton a federally-funded exploration of the West, but to no avail. Smith informed Eaton that he was completing a map of the West derived from his own journeys. In May, Smith and his partners launched a planned paramilitary trading party to Santa Fe. On May 27, while searching for water in present-day southwest Kansas, Smith disappeared. It was learned weeks later that he had been killed during an encounter with a Comanche defense party– his body was never recovered.

After his death, Smith and his accomplishments were mostly forgotten by Americans. At the beginning of the 20th century, scholars and historians made efforts to recognize and study his achievements. In 1918, a book by Harrison Clifford Dale was published covering Ashley-Smith's western explorations. In 1935, Smith's summary autobiography was finally listed in a biographical dictionary. Smith's first comprehensive biography by Maurice S. Sullivan was published in 1936. A popular Smith biography by Dale Morgan, published in 1953, established Smith as an authentic national hero. Smith's map of the West in 1831 was used by the U.S. Army, including western explorer John C. Frémont, during the early 1840s.

Smith was born in Jericho, now Bainbridge, Chenango County, New York, on January 6, 1799, to Jedediah Smith I, a general store owner from New Hampshire, and Sally Strong, both of whom were descended entirely from families that came to New England from England during the Puritan emigration between 1620 and 1640. Smith received adequate English instruction, learned some Latin, and was taught how to write decently. Around 1810, Smith's father was caught up in a legal issue involving counterfeit currency after which the elder Smith moved his family west to Erie County, Pennsylvania.

At age 13, Smith worked as a clerk on a Lake Erie freighter, where he learned business practices and probably met traders returning from the far west to Montreal. This work gave Smith an ambition for adventurous wilderness trade. According to Dale L. Morgan, Smith's love of nature and adventure came from his mentor, Dr. Titus G. V. Simons, a pioneer medical doctor who was on close terms with the Smith family. Morgan speculated that Simons gave the young Smith a copy of Meriwether Lewis' and William Clark's 1814 book of their 1804–1806 expedition to the Pacific, and according to legend Smith carried this journal on all of his travels throughout the American West. Smith provided Clark, who had become superintendent of Indian affairs, much information from his own expeditions to the West. In 1817, the Smith family moved westward to Ohio and settled in Green Township in what is present-day Ashland County.

Coming from a family of modest means, Smith sought to make his own way. He may have left his family in search of a trade or employment a year prior to their settlement in Green Township. In 1822, Smith was living in St. Louis. The same year Smith responded to an advertisement in the Missouri Gazette placed by General William H. Ashley. General Ashley and Major Andrew Henry, veterans of the War of 1812, had established a partnership to engage in the fur trade and were looking for "One Hundred" "Enterprising Young Men" to explore and trap in the Rocky Mountains. Superintendent of Indian Affairs William Clark had granted Ashley and Henry license to trade with Native Americans in the upper Missouri River, and he actively encouraged them to compete with the powerful British fur trade in the Pacific Northwest. Smith, a 6-foot-tall, 23-year-old with a commanding presence, impressed General Ashley to hire him. In late spring, Smith started up the Missouri on the keelboat Enterprize, which sank three weeks into the journey. Smith and the other men waited at the site of the wreck for a replacement boat, hunting and foraging for food. Ashley brought up another boat with an additional 46 men and upon proceeding upriver, Smith got his first glimpse of the western frontier, coming into contact with the Sioux and Arikara. On October 1, Smith reached Fort Henry at the mouth of the Yellowstone River, which had just been built by Major Henry and the men that he had led up earlier. Smith and some other men continued up Missouri River to the mouth of the Musselshell River, where they built a camp from which to trap through the winter.

In the spring of 1823, Major Henry ordered Smith back down the Missouri River to the Grand River with a message for Ashley to buy horses from the Arikaras, who, because of a recent skirmish with Missouri Fur Company men, were antagonistic to the white traders. Ashley, who was bringing supplies as well as 70 new men upriver by boat, met Smith at the Arikara village on May 30. They negotiated a trade for several horses and 200 buffalo robes and planned to leave as soon as possible to avert trouble, but weather delayed them. Before they could depart, an incident provoked an Arikara attack. Forty Ashley men, including Smith, were caught in a vulnerable position, and 12 were killed in the ensuing battle. Smith's conduct during the defense was the foundation of his reputation: "When his party was in danger, Mr. Smith was always among the foremost to meet it, and the last to fly; those who saw him on shore, at the Riccaree fight, in 1823, can attest to the truth of this assertion."