Fort Pontchartrain du Détroit or Fort Detroit (1701–1796) was a French and later British fortification established in 1701 on the north side of the Detroit River by Antoine Laumet de Lamothe Cadillac. A settlement based on the fur trade, farming and missionary work slowly developed in the area. The fort was located in what is now downtown Detroit, northeast of the intersection of Washington Boulevard and West Jefferson Avenue.

Fort Pontchartrain du Détroit was attacked by the Meskwaki during the Fox Wars, and was the target of an aborted attack by English-aligned Wyandot during King George's War. During the French and Indian War, Fort Pontchartrain du Détroit surrendered to the British on November 29, 1760 after the capture of Montreal. It was besieged by Indigenous forces during Pontiac's War in 1763. The British controlled the area throughout the American Revolutionary War, but replaced the French fort with the newly constructed Fort Lernoult in 1779. While the territory on what is now the Michigan side of the Detroit River was ceded to the United States in the Treaty of Paris in 1783, control of the fort was not transferred until 1796, after the Jay Treaty.

The river flowing between Lake St. Clair and Lake Erie was called by Le Détroit du Lac Érié by the French, meaning "The Strait of Lake Erie." In 1698, Antoine Laumet de Lamothe Cadillac, who had previously commanded Fort de Buade at Michilimackinac, proposed the establishment of a colony at Detroit. French families would be recruited as settlers, and the Indigenous tribes living near Michilimackinac would be encouraged to migrate to the area. The settlement would not only prevent English expansion into the Pays d'en Haut (Upper Country), but would also deter Iroquois aggression. Jérôme Phélypeaux de Pontchartrain, the French Secretary of State of the Navy, approved the plan despite the misgivings of New France's Governor and Intendant.

In June 1701, Cadillac set out from Lachine near Montreal with 100 settlers and soldiers. The expedition followed a northerly route up the Ottawa River and across to Georgian Bay and Lake Huron. The expedition reached Grosse Ile on the Detroit River on July 23. The following day, the expedition returned upstream several miles to a bluff on the north shore of the river at its narrowest point. Cadillac commenced the construction of a fort at this location which he named Fort Pontchartrain du Détroit. The first building completed was a chapel dedicated to Saint Anne, the patron saint of New France. In September, the first two European women arrived at the fort: Cadillac's wife, Marie-Thérèse Guyon, and Marie Anne Picoté de Belestre, the wife of Cadillac's lieutenant, Alphonse de Tonty.

Fort Pontchartrain du Détroit was built from white oak and initially enclosed an area of about 0.85 acres (0.34 hectares). The palisade was roughly 12 feet (3.7 m) tall with a bastion positioned at each corner. Dwellings, a warehouse, and the chapel were constructed inside the fort. For many years the entire European population lived within the palisade. In October 1703, a fire destroyed the chapel and the house of the Recollect priest, Constantin Delhalle, as well as the residences of Cadillac and Tonty.

After the fort was established, Odawa (Ottawa) from Michilimackinac, and Wyandot (Huron) from Michilimackinac and the St. Joseph River migrated to Detroit and established palisaded villages. Groups of Miami, Ojibwe and later Potawatomi also migrated to the area. In 1705, Cadillac reported an Indigenous population at Detroit of 2,000.

In June 1706, while Cadillac was at Quebec, Odawa warriors at Detroit organized an expedition against the Sioux. As they were setting out, a Potowatomi who had married a Miami woman mistakenly warned the Odawa that the Miami were planning to raid their village during their absence. The Odawa chief known as Le Pesant or "The Bear" decided to turn back and lead a preemptive strike. They surprised eight Miami near the fort and slew seven of them. Étienne de Veniard, Sieur de Bourgmont, who was commanding Fort Pontchartrain in Cadillac's absence, provided sanctuary to the Miami and ordered his men to open fire on the Odawa. Father Delhalle and a soldier were caught outside the fort and were killed. In the series of raids, ambushes, and counter-attacks that followed, the Miami were joined by the Wyandot. About 30 Odawa, 50 Miami and an unknown number of Wyandot were killed. The Odawa abandoned their village and moved back to Michilimackinac but returned by 1708.

Contemporary accounts, both Indigenous and French, do not agree on the cause of the attack or who was to blame. Bourgmont was criticized for his handling of the incident and deserted after Cadillac's return. For the next several years, he lived as a coureur des bois before undertaking an exploration of the Missouri River in 1714. The Governor General of New France, Philippe de Rigaud, Marquis de Vaudreuil concluded that Le Pesant was responsible. Vaunreuil insisted that Le Pesant be turned over to the French, and gave Cadillac the authority to arrest and execute him. Le Pesant was apprehended at Michilimackinac and brought to Detroit but was allowed to escape. Angry that Le Pesant had not been executed, the Miami and Wyandot murdered three settlers in the vicinity of the fort. Cadillac later led a lackluster attack against the Miami settled on the St. Joseph River.