Luther Sage "Yellowstone" Kelly (July 27, 1849 – December 17, 1928) was an American soldier, hunter, scout, adventurer and administrator. He served briefly in the American Civil War and then in an 1898 expedition to Alaska. He commanded a U.S. Army company in the Philippine–American War and later served in the civilian administration of the Philippines. On June 26, 1929, Yellowstone Kelly was buried with full military honors overlooking the Yellowstone Valley in Billings, Montana.

Luther Sage Kelly was born July 27, 1849, in Geneva, New York. His father, also named Luther Kelly, owned a drug and grocery store in Geneva. His mother, Jeanette Eliza Sage, was the daughter of Colonel Hezekiah Sage of nearby Chittenango. Kelly's father died on February 14, 1857, leaving him the man of the family, but the family had enough money saved to live comfortably. In either late 1864 or early 1865, Kelly entered the Genesee Wesleyan Seminary, but his real interest was in joining the army and fighting in the Civil War; he later wrote that he "deplored the fact" that his youth rendered him unfit for military service at that time.

In the spring of 1865, with the Civil War winding down, Kelly secured permission from his mother to join the Army. He traveled to Rochester, New York, where he attempted to join the Fourth New York Cavalry but was turned down due his young age (15). Later he joined the 10th Infantry by lying about his age. He was unaware that the 10th Infantry was not a volunteer corps and that he would be obliged to continue serving after the war.

Kelly was sent to City Point, Virginia. After Robert E. Lee's surrender at Appomattox his regiment was sent to Richmond and he marched towards Washington, D.C., encamping south of the Potomac River until after the Grand Review of the Armies. Because Kelly's unit had not participated in the Grand Review, it was selected for a parade through Washington on June 8, during which Kelly served as part of the guard detail for the reviewing officer, "his first official duty of any real consequence".

Kelly's unit was stationed in Washington over the summer. In November they were moved by train to St. Paul, Minnesota to be stationed at Fort Snelling for the winter. In May, his company moved among Fort Abercrombie, Fort Wadsworth and Fort Ransom, all in the Dakota Territory. During his free time at these forts, Kelly hunted game to provide fresh meat for his fellow soldiers. In April 1868, Kelly's enlistment in the Army ended, and he was discharged at Fort Ransom.

After leaving the army, Kelly embarked on what The New York Times later called "the most adventurous period of his life", establishing himself as "one of the greatest hunters, trappers, and Indian scouts" of the American West. He first traveled to Fort Garry, now Winnipeg in Canada, where he joined a group of miners, traveling with them to the Red River, where he spent the winter. He left the miners to cross the Assiniboine River, falling in with a group headed toward the Mouse River. After meeting Sitting Bull with this group, Kelly headed alone toward the Missouri River, eventually reaching Fort Buford in the winter.

Not long after his arrival at Fort Buford, Kelly volunteered to carry dispatches to Fort Stevenson, approximately fifty miles down the Missouri River. He left the fort on February 5, 1869. The route between the forts was considered so dangerous, due to the presence of Sioux warriors, that mail carriers were generally accompanied by a cavalry escort, but Kelly set out alone. He arrived safely at Fort Stevenson then set out on his return journey, spending the night at the camp of Bloody Knife, an Arickaree chieftain. The next morning, Kelly was ambushed by two Sioux warriors. The first wounded Kelly's horse with a rifle, while the second shot Kelly in the knee with an arrow. Kelly managed to shoot and kill the first attacker quickly, but the second took cover behind a tree. Kelly eventually shot and killed his second assailant, then returned to Bloody Knife's camp to tell the story. Kelly spent a few days at Bloody Knife's camp recovering from his wound, then rode back to Fort Buford, becoming "something of a hero and a local celebrity" for defeating his two assailants.

(A lot left out here) By his own admission, in his memoirs Kelly noted that he roamed the Judith Basin of Montana for many years especially during that crucial period between 1870 and 1880 when the Indian Wars were heating up. He became renowned as both a hunter and a scout, hunting elk, buffalo, antelope. In fact, he adopted a baby antelope in the late spring of 1876 and was bringing it back to his ranch in the Basin when news of the Little Big Horn reached him. Leaving the antelope with friends he rejoined the Army. He was a prominent scout under General Crook, and was assigned to lead Colonel Nelson Miles' 5th Infantry in Crook's pursuit of Sitting Bull and Crazy Horse.