The siege of Fort Pitt took place during June and July 1763 in what is now the city of Pittsburgh, Pennsylvania, United States. The siege was a part of Pontiac's War, an effort by Native Americans to remove the Anglo-Americans from the Ohio Country and Allegheny Plateau after they refused to honor their promises and treaties to leave voluntarily after the defeat of the French. The Native American efforts of diplomacy, and by siege, to remove the Anglo-Americans from Fort Pitt ultimately failed.

This event is known for a possible attempt at biological warfare, in which William Trent and Simeon Ecuyer, a Swiss mercenary in British service, may have given items from a smallpox infirmary as gifts to Native American emissaries with the hope of spreading the deadly disease to nearby tribes. The effectiveness is unknown, although it is known that the method used is inefficient compared to respiratory transmission and these attempts to spread the disease are difficult to differentiate from epidemics occurring from previous contacts with colonists.

Fort Pitt was built in 1758 during the French and Indian War, on the site of what was previously Fort Duquesne in what is now the city of Pittsburgh, Pennsylvania, United States. The French abandoned and destroyed Fort Duquesne in November 1758 with the approach of General John Forbes's expedition. The Forbes expedition was successful in part because of the Treaty of Easton, in which area American Indians agreed to end their alliance with the French. American Indians—primarily the Six Nations, Delawares and Shawnees—made this agreement with the understanding that the British would leave the area after their war with the French. Instead of leaving the territory west of the Appalachian Mountains as they had agreed, the Anglo-Americans remained on Native lands and reinforced their forts while settlers continued to push westward, despite the Royal Proclamation of 1763 placing a limit upon the westward expansion of the American colonies. The hostilities between the French and British declined significantly after 1760, followed by a final cessation of hostilities and the formal surrender of the French at the Treaty of Paris in February 1763.

The attacks led by Pontiac against the British in early May 1763, near Fort Detroit, mark what is generally considered to be the beginning of Pontiac's War. The siege of Fort Pitt and numerous other British forts during the spring and summer of 1763 were part of an effort by American Indians to reclaim their territory by driving the British out of the Ohio Country and back across the Appalachian Mountains. While many of the forts and outposts in the region were destroyed, the Indian effort to remove the British from Fort Pitt ultimately failed.

By May 27, the uprising reached the tribes near Fort Pitt, and there were many signs of impending hostilities. The captain of the Fort Pitt militia learned that the Delaware tribe just north of the fort had abandoned their dwellings and cornfields overnight. The Mingo had also abandoned their villages further up the river. The proprietor of the Pennsylvania provincial store reported that numerous Delaware warriors had arrived "in fear and haste" to exchange their skins for gunpowder and lead. The western Delaware warrior leaders Wolf and Keekyuscung had fewer than 100 warriors, so did not immediately attack the well-fortified Fort Pitt. Instead, on May 29, they attacked the supporting farms, plantations and villages in the vicinity of the fort. Panicked settlers crowded into the already overcrowded fort. Captain Simeon Ecuyer, a 22-year veteran Swiss mercenary in British service, tried to ready his fort after this news of expanding hostilities, putting his 230 men, half regulars and half quickly organized militia, on alert. The fort's exceptional structural defenses, made of stone with bastions covering all angles of attack, were supported by 16 cannons which he had permanently loaded. Ecuyer demolished the nearby village houses and structures to deny cover for attackers. He had trenches dug outside the fort, and set out beaver traps. Smallpox had been discovered within the fort, prompting Ecuyer to build a makeshift hospital in which to quarantine those infected.

On the June 16, four Shawnee visited Fort Pitt and warned Alexander McKee and Captain Simeon Ecuyer that several Indian nations had accepted Pontiac's war belt and bloody hatchet and were going on the offensive against the British, but that the Delaware were still divided, with the older Delaware chiefs advising against war. The following day, however, the Shawnee returned and reported a more threatening situation, saying that all the nations "had taken up the hatchet" against the British, and were going to attack Fort Pitt. Even the local Shawnee themselves "were afraid to refuse" to join the uprising, a subtle hint that the occupants of Fort Pitt should leave. Ecuyer dismissed the warnings and ignored the requests to leave. On June 22, Fort Pitt was attacked on three sides by Shawnee, western Delaware, Mingo and Seneca, which prompted return fire from Ecuyer's artillery. This initial attack on the fort was repelled. Since the Indians were unfamiliar with siege warfare, they opted to try diplomacy yet again. On June 24, Turtleheart spoke with McKee and Trent outside the fort, informing them that all of the other forts had fallen, and that Fort Pitt "is the only one you have left in our country." He warned McKee that "six different nations of Indians" were ready to attack if the garrison at the fort did not retreat immediately. They thanked Turtleheart and assured him that Fort Pitt could withstand "all nations of Indians", and they presented the Indian dignitaries with two small blankets and a handkerchief from the smallpox hospital. For the next several days it remained relatively quiet, although reports were coming in about fort after fort falling before large bands of attacking warriors.

July 3, four Ottawa newcomers requested a parley and tried to trick the occupants of Fort Pitt into surrender, but the ruse failed. This was followed by several weeks of relative quiet, through July 18 when a large group of warriors arrived, likely from the Fort Ligonier area. McKee was informed by the Shawnee that the Indians were still hopeful of an amicable outcome, similar to agreements just made at Detroit. On July 26, a large conference headed by Ecuyer was convened with several leaders of the Ohioan tribes outside the walls of Fort Pitt. The Indian delegation, Shingas, Wingenum and Grey Eyes among them, came to the fort under a flag of truce to parley, and again requested that the British leave this place. They explained that by taking the Indian's country the British caused this war, and Tessecumme of the Delaware noted that the British were the cause of the trouble since they had broken their promises and treaties. They had come onto Indian land and built forts, despite being asked not to, so now the tribes in the area have amassed to take back their lands. He informed Ecuyer that there was still a short time remaining to leave peacefully. The Delaware and Shawnee chiefs made sure Captain Ecuyer at Fort Pitt understood the cause of the conflict. Turtleheart told him, "You marched your armies into our country, and built forts here, though we told you, again and again, that we wished you to move, this land is ours, and not yours." The Delaware also let it be known, "that all the country was theirs; that they had been cheated out of it, and that they would carry on the war till they burnt Philadelphia". The British refused to leave, claiming that this was their home now. They bluffed that they could hold out for three years, and bragged that several large armies were coming to their aid. This "very much enraged" the Indian delegation, Trent wrote, "White Eyes and Wingenum seemed to be very much irritated and would not shake hands with our people at parting." On July 28, the siege began in earnest and continued for several days. Seven of the fort garrison were wounded, at least one mortally; Ecuyer was wounded in the leg by an arrow.

For Commander-in-Chief, North America Jeffery Amherst, who before the war had dismissed the possibility that the Indians would offer any effective resistance to British rule, the military situation over the summer had become increasingly grim. The frustration was so great, he wrote to Colonel Henry Bouquet and instructed him not to take any Indian prisoners. He proposed that they should be intentionally exposed to smallpox, hunted down with dogs, and "Every other method that can serve to Extirpate this Execrable Race." Amherst had directed Bouquet to take his troops to relieve Fort Pitt, a march that would take several weeks. At Fort Pitt, the siege didn't let up until August 1, 1763, when most of the Indians broke off their attack in order to intercept the body of almost 500 British troops marching to the fort under Colonel Bouquet. On August 5, these two forces met at Edge Hill in the Battle of Bushy Run. Bouquet survived the attack and the Indians were unable to prevent his command from relieving Fort Pitt on August 10.