John Charles Frémont or Fremont (January 21, 1813 – July 13, 1890) was an American explorer, military officer, and politician. He was a U.S. Senator from California, and, in 1856, was the first Republican nominee for president of the United States and founder of the California Republican Party when he was nominated.

A native of Georgia, he was an opponent of slavery. In the 1840s, Frémont led five expeditions into the Western United States. While on the third expedition, he and his men committed a number of massacres against Native Americans in California. During the Mexican–American War, Frémont, a major in the U.S. Army, took control of California from the California Republic in 1846. Frémont was convicted in court-martial for mutiny and insubordination after a conflict over who was the rightful military governor of California. After his sentence was commuted and he was reinstated by President Polk, Frémont resigned from the Army. Afterwards, Frémont settled in California at Monterey while buying cheap land in the Sierra foothills. When gold was found on his Mariposa ranch, Frémont became a wealthy man during the California Gold Rush. Frémont became one of the first two U.S. senators elected from the new state of California in 1850. Frémont was the first presidential candidate of the new Republican Party, carrying most of the North. He lost the 1856 presidential election to Democrat James Buchanan when Know Nothings split the vote.

In 1844, John C Frémont received orders to lead another expedition to find a more southern route across the Rockies and explore the area south of the Columbia River. Frémont would complete his official orders and more, crossing into the Mexican province of California.

Frémont's successful first expedition led quickly to a second; it began in the summer of 1843. The more ambitious goal this time was to map and describe the second half of the Oregon Trail, find an alternate route to the South Pass, and push westward toward the Pacific Ocean on the Columbia River in Oregon Country. Frémont and his almost 40 well-equipped men left the Missouri River in May after he controversially obtained a 12-pound howitzer cannon in St. Louis. Frémont invited Carson on the second expedition, due to his proven skills, and he joined Frémont's party on the Arkansas River. Unable to find a new route through Colorado to the South Pass, Frémont took to the regular Oregon Trail, passing the main body of the great immigration of 1843. His party stopped to explore the northern part of the Great Salt Lake, then traveling by way Fort Hall and Fort Boise to Marcus Whitman's mission, along the Snake River to the Columbia River and in to Oregon. Frémont's endurance, energy, and resourcefulness over the long journey was remarkable. Traveling west along the Columbia, they came within sight of the Cascade Range peaks and mapped Mount St. Helens and Mount Hood. Reaching the Dalles on November 5, Frémont left his party and traveled to the British-held Fort Vancouver for supplies.

Rather than turning around and heading back to St. Louis, Frémont resolved to explore the Great Basin between the Rockies and the Sierras and fulfill Benton's dream of acquiring the West for the United States. Frémont and his party turned south along the eastern flank of the Cascades through the Oregon territory to Pyramid Lake, which he named. Looping back to the east to stay on the eastern side of the Sierra Nevada mountain range, they turned south again as far as present-day Minden, Nevada, reaching the Carson River on January 18, 1844. From an area near what later became Virginia City, Frémont turned west into the cold and snowy Sierra Nevada, becoming one of the first Americans to see Lake Tahoe. Carson successfully led Frémont's party through a new pass over the high Sierras, which Frémont named Carson Pass in his honor. Frémont and his party then descended the American River valley to Sutter's Fort (Spanish: Nueva Helvetia) at present-day Sacramento, California, in early March. Captain John Sutter, a German-Mexican (and later American by treaty) immigrant and founder of the fort, received Frémont gladly and refitted his expedition party. While at Sutter's Fort, Frémont talked to American settlers, who were growing numerous, and found that Mexican authority over California was very weak.

Leaving Sutter's Fort, Frémont and his men headed south following Smith's trail on the eastern edge of the San Joaquin Valley until he struck the "Spanish Trail" between Los Angeles and Santa Fe, and headed east through Tehachapi Pass and present-day Las Vegas before regaining Smith's trail north through Utah and back to South Pass. Exploring the Great Basin, Frémont verified that all the land (centered on modern-day Nevada between Reno and Salt Lake City) was endorheic, without any outlet rivers flowing towards the sea. The finding contributed greatly to a better understanding of North American geography, and disproved a longstanding legend of a 'Buenaventura River' that flowed out the Great Basin across the Sierra Nevada. After exploring Utah Lake, Frémont traveled by way of the Pueblo until he reached Bent's Fort on the Arkansas River. In August 1844, Frémont and his party finally arrived back in St. Louis, enthusiastically received by the people, ending the journey that lasted over one year. His wife Jessie and Frémont returned to Washington, where the two wrote a second report, scientific in detail, showing the Oregon Trail was not difficult to travel and that the Northwest had fertile land.[24] Senator Buchanan ordered the printing of 10,000 copies to be used by settlers and fervor the popular movement of national expansion.