Carlisle Fort, also known as Fort Carlisle, the Fort at Carlisle, Fort Lowther or Fort Louther, was a stockade built in the town of Carlisle, Pennsylvania during the French and Indian War, for local defense. The fort also served as a supply depot and a military headquarters during the Forbes Expedition in 1758. It was one of the first forts authorized for construction by Governor Robert Hunter Morris in 1755, although construction took almost two years to complete. It was never attacked, and was abandoned after 1758.

At the beginning of the French and Indian War, Braddock's defeat at the Battle of the Monongahela left Pennsylvania without a professional military force. Lenape chiefs Shingas and Captain Jacobs launched dozens of Shawnee and Delaware raids against British colonial settlements, killing and capturing hundreds of colonists and destroying settlements across western and central Pennsylvania. In late 1755, Colonel John Armstrong wrote to Governor Robert Hunter Morris: "I am of the opinion that no other means of defense than a chain of blockhouses along or near the south side of the Kittatinny Mountains from the Susquehanna to the temporary line, can secure the lives and property of the inhabitants of this country."

A temporary stockade had been built in Carlisle before 1753, and was briefly garrisoned with about a dozen militia. In May 1753, John O'Neal wrote to Governor James Hamilton describing this stockade: "The Garrison here consists only of twelve men. The Stockade originally occupied two acres of ground square, with a blockhouse in each corner; these buildings are now in ruin."

Attacks by Native Americans led authorities to propose a more permanent structure in which residents could take refuge during an attack. Following Braddock's Defeat in early July 1755, Governor Morris spent several weeks in Carlisle, supervising the defenses and the construction of the fort, writing on July 17, 1755: "I have laid out a place in the middle of this town which the inhabitants intend to fortify with logs as a retreat for their women and Children in case they should be attacked." On July 31 he wrote to Thomas Penn: "I...returned to Philadelphia having at the request of the people laid the Ground for a Wooden Fort in the Town of Carlisle...the people being much disheartend and inclining to quit their plantations, I encouraged them to Act with resolution in their own defence and formed four Companies of Militia to whom I distributed some Powder and Lead."

The fort was built on the west bank of LeTort Spring Run, which runs through Carlisle. An 1841 history of Carlisle describes the fort:

A 1758 plan of the fort shows it as a square about 130 feet (40 m) on each side, with bastions at each corner. Fourteen unidentified buildings are depicted inside the stockade.

While the fort was under construction, the older stockade was garrisoned by fifty militia recruited by Governor Morris in July, however they were disorganized and unreliable, according to a letter from John Smith to Isaac Norris on November 3, 1755: "We have built a Small Stockade here but it will avail but little in Case an Enemy appears, having neither Order nor any One that can be depended on, every one fearing the other will flee in Case of an attack."

In October and November a large quantity of powder, lead and gunflints was sent to the garrison, as well as 150 rifles, "for the use of the Inhabitants of Carlisle." As construction continued, two swivel guns with powder and lead were sent to Carlisle on April 6, 1756. On April 16, a detachment of 30 provincial regulars was temporarily posted in Carlisle, but they were transferred to Harris' Ferry in May. Lieutenant Colonel John Armstrong recruited a company of troops for Carlisle, and they began ranging the territory between Carlisle and Fort Augusta.