The Treaty of Fort Pitt, also known as the Treaty With the Delawares, the Delaware Treaty, or the Fourth Treaty of Pittsburgh, was signed on September 17, 1778, and was the first written treaty between the new United States of America and any American Indians, in this case the Lenape (Delaware Indians). Although many informal treaties were held with Native Americans during the American Revolution from 1775 to 1783, the only one that resulted in a formal document was signed at Fort Pitt, Pennsylvania, now the site of Downtown Pittsburgh. It was essentially a formal treaty of alliance.

The treaty gave the United States permission to travel through the Delaware territory and called for the Delawares to afford American troops whatever aid they might require in their war against Great Britain, including the use of their own warriors. The United States was planning to attack the British fort at Detroit, and Lenape friendship was essential for success.

In exchange, the United States promised "articles of clothing, utensils and implements of war" and to build a fort in Delaware country "for the better security of the old men, women and children... whilst their warriors are engaged against the common enemy." Although not part of the written treaty, the commissioners pointed out the American alliance with France and intended that the Delaware would become active allies in the war against the British.

According to Daniel Richter in Facing East from Indian Country, the Delaware perceived the agreement "merely as free passage" of revolutionary troops and the building of a protective fort for the defending white settlers. The American leaders intended to use the fort for offensive campaigns and wrote into the treaty that the Delaware would attack their native neighbors.

The treaty also recognized the Delawares as a sovereign nation, guaranteed their territorial rights, and even encouraged the other Ohio Country Indian tribes friendly to the United States to form a state headed by the Delawares with representation in the Continental Congress. The extraordinary measure had little chance of success, and some suggest that the authors of the treaty were knowingly dishonest and deceitful. Others suggest that it was the Delaware chief White Eyes who proposed the measure in the hope that the Delaware and other tribes might become the fourteenth state of the United States. In any case, it was never acted upon by either the United States or the Delaware Indians.

Within a year, the Delaware were expressing grievances about the treaty. A delegation visited Philadelphia in 1779 to explain its dissatisfaction to the Continental Congress, but nothing changed, and the peace between the United States and the Delaware Indians collapsed. White Eyes, the tribe's most outspoken ally of the United States, died of a smallpox, and the tribe soon joined the British in the war against the American revolutionaries.

Signers of the treaty were White Eyes, Captain Pipe (Hopocan), and John Kill Buck (Gelelemend) for the Lenape, and Andrew Lewis and Thomas Lewis for the Americans. Witnesses included Brigadier General Lachlan McIntosh, Colonel Daniel Brodhead, and Colonel William Crawford.

The 2013 Sacagawea dollar commemorates the Treaty of Fort Pitt. The coin depicts a turkey, a howling wolf, and a turtle, symbols of the Lenape. Its design was created by Susan Gamble, as part of the Artistic Infusion Program, and engraved by Phebe Hemphill.