Thomas Gaddis (December 28, 1742 – June 10, 1834) was a militia officer in the American Revolutionary War. He was born in Winchester, Frederick County, Virginia, and married Hannah Rice in 1764; the same year he built Fort Gaddis, a refuge from attacks by Native Americans, located on the Catawba Trail. In fact, Pennsylvania and Virginia had conflicting claims in the area Gaddis settled. Though he maintained his loyalty to Virginia, Gaddis also protected his investment by recording his patent with Pennsylvania authorities. By 1773, both states created new geopolitical boundaries in recognition of increased white settlement. Pennsylvania formed Westmoreland County out of the larger Bedford County, and Virginia established the District of West Augusta. In 1776, West Augusta was further divided into three counties: Ohio, Yohogania, and Monongalia, where Gaddis and his family resided.

Thomas Gaddis was appointed captain of the Monongalia County militia on August 23, 1776. By September 9th, he had raised a company of militia and proceeded to build what was afterward known as Fort Liberty in Monongalia County (two miles south of present-day Uniontown, PA, at Fort Gaddis), where they were stationed from September 1776 to January 1777. However, some historical texts have confused this fort with another Fort Liberty that was located along the Ohio River (present-day West Liberty, West Virginia) and a fort located at Beech Bottom, West Virginia, about ten miles north of Fort Henry (West Virginia), which also was constructed by Gaddis and his militia company. On February 17, 1777, Gaddis was commissioned as lieutenant colonel of Monongalia County by Patrick Henry, Governor of Virginia, and took command of Prickett's Fort, Fort Scott, Fort Stradler, Fort Jackson, and Fort Lindley. Shortly afterward, he was promoted to full colonel.

In August 1777, Gaddis and Colonel Zackquill Morgan learned that a substantial number of settlers in the Redstone area, a region south of Pittsburgh, had taken an oath of allegiance to the Monarchy of the United Kingdom and were plotting on Great Britain's behalf. Gaddis informed Lieutenant Colonel Thomas Brown at Redstone Old Fort, on the Monongahela River, requesting him to place an extra guard on the powder magazine. In a dispatch to Brown, he wrote:

Dear Sir,

A certain person was at my house on Monday the 25th last, and he made oath to me that the Tories have joined themselves together for to cut off the inhabitants, and we know not what hour they will rise. Therefore it would be proper that you would take a particular care and keep a strong guard over the Magazine for a few days, till we can use some means with them. This day I am starting with a party of men for to succor the people and suppress the Tories. I would desire that you would do your utmost endeavor and warn the friends of our country to be upon their watch.

Sir, I remain respectfully your friend,

Thos. Gaddis

The loyalists planned to seize the magazine at Redstone, but Brown mustered a guard of fifteen men, and a militia force of a hundred patriots under Gaddis and Captain Henry Enoch captured twelve tories and scattered the remainder. The prisoners were escorted to Virginia's capital in Williamsburg, took an oath of allegiance, and eventually returned home.