Sidney Edgerton (August 17, 1818 – July 19, 1900) was an American politician, lawyer, judge and teacher from Ohio. He served during the American Civil War, as a Squirrel Hunter. During this time, Edgerton served as a U.S. Congressman. In 1863, Abraham Lincoln appointed him the first Chief justice of the Idaho Territorial Supreme Court. Edgerton lobbied for the creation of separate territories, out of the Idaho Territory, and in 1864, Abraham Lincoln appointed Edgerton as the first Territorial Governor of Montana. During his term as Territorial Governor, he was an alleged member of the infamous Montana Vigilantes, and was reputedly among its founders.

He was a sickly child that was not expected to survive; burial clothing was ordered for him. He survived and, eventually, moved to Ohio. He became a lawyer, and was involved in both the Free Soil Party and the Republican Party. After John Brown's raid on Harpers Ferry, Edgerton was invited, by Brown's family, to settle Brown's affairs. He never was able to meet with Brown. He had a successful career as a politician, and after his term ended in the Territory of Montana, Edgerton returned to Ohio. He served as a lawyer in his home state until his death in 1900.

Edgerton was born in Cazenovia, New York, on August 17, 1818. His parents were Amos and Zerviah (Graham) Edgerton, both of Connecticut. Zerviah was a cousin of millionaire philanthropist Anson Greene Phelps. As a child, Edgerton was so sickly and frail that burial clothes were prepared for him. Whereas the young Edgerton avoided death, the same could not be said for his father, a teacher by trade who had been blind for years. Amos Edgerton died when Sidney was still an infant. Left with six children to raise, Zerviah Edgerton struggled to maintain her family. By age eight, young Sidney was out of the home and working himself through school. He eventually attended the Genesee Wesleyan Seminary in Lima, New York, where his cousin taught, and where he would later become an instructor as well.

In 1844, he moved to Ohio and began working in the law office of Rufus P. Spalding. Edgerton also taught in an academy in Tallmadge, Ohio, the same year. He studied law and graduated from the Cincinnati Law School in 1845. The next year Edgerton was admitted to the bar and began the practice of law in Akron, Ohio. During this time, Edgerton declared himself an Agnostic. He married Mary Wright (1827–1885) of Tallmadge on May 18, 1848, but Mary almost ended the relationship, because of Edgerton's religious views.

Edgerton was a delegate to the convention that formed the Free Soil Party, in 1848. He then served as prosecuting attorney of Summit County, Ohio, from 1852 to 1856. Later in 1856, Edgerton was a delegate to the first Republican National Convention. That same year, he was nominated for probate judge, but he declined the nomination. Edgerton was elected as a Republican to the United States House of Representatives in 1858.

Edgerton began his House term in 1859. As an abolitionist, he was at risk of attack; when his term began, he purchased a sword for his defense. The sword was held, secretly, inside a walking cane. As an ardent anti-slavery member of the House of Representatives, Edgerton made numerous speeches about its abolition. After John Brown's raid on Harpers Ferry, Edgerton was asked by Brown's family to come and settle his affairs. This was very dangerous, as Edgerton was anti-slavery. Edgerton went by train, and was joined by Congressman Alexander Boteler and Congressman H. G. Blake. While on the Baltimore & Ohio Railroad, Boteler was told that the men did not need to go on. Boteler and Blake listened to the advice, but Edgerton refused to go back.

On his arrival at Charles Town, Edgerton found the commander of Harper's Ferry, General William Taliaferro. Edgerton told Taliaferro about his request, but Taliaferro informed him that Governor Wise would only allow family and a minister to visit John Brown. That night, Taliaferro arranged for a wagon to take him back to Washington, D.C. Edgerton was given a driver and a guard, for his journey. During his ride back, a group of men on horses rode up, and Edgerton's guard jumped out of the wagon and ran. The men on horses retrieved the man, and brought him back to the wagon. Edgerton asked his guard why he ran, and the guard replied, "I heard them say that they would kill [you]". The men on horses then attempted to get Edgerton to leave the wagon, but he refused. He eventually made it out of Virginia, but always believed had he left the wagon, the men would have killed him. After this encounter, Edgerton viewed Southerners, generally, with contempt. Edgerton continued to be a large voice in the anti-slavery movement, but decided not to run for reelection in 1862.

During the Civil War, Edgerton served briefly as colonel in the Ohio Militia. Edgerton was one of the Squirrel Hunters, expert shots from Ohio, and served at the Defense of Cincinnati. Edgerton served as both a U.S. Congressman and soldier at the same time, during the first few years of the war.