Sarnia is a city in Lambton County, Ontario, Canada. It had a 2021 population of 72,047, and is the largest city on Lake Huron. Sarnia is located on the eastern bank of the junction between the Upper and Lower Great Lakes, where Lake Huron flows into the St. Clair River in the Southwestern Ontario region, which forms the Canada–United States border, directly across from Port Huron, Michigan.

The site's natural harbour first attracted the French explorer La Salle. He named the site "The Rapids" on 23 August 1679, when he had horses and men pull his 45-ton barque Le Griffon north against the nearly four-knot current of the St. Clair River. This was the first time that a vessel other than a canoe or other oar-powered vessel had sailed into Lake Huron, and La Salle's voyage was germinal in the development of commercial shipping on the Great Lakes. Located in the natural harbour, the Sarnia port remains an important centre for lake freighters and oceangoing ships carrying cargoes of grain and petroleum products. The natural port and the salt caverns that exist in the surrounding areas, together with the oil discovered in nearby Oil Springs in 1858, led to the dramatic growth of the petroleum industry in this area. Because Oil Springs was the first place in Canada and North America to drill commercially for oil, the knowledge that was acquired there resulted in oil drillers from Sarnia travelling the world teaching other enterprises and nations how to drill for oil.

The complex of refining and chemical companies is called "Chemical Valley" and located south of downtown Sarnia. In 2011, the city had the highest level of particulates air pollution of any Canadian city, but it has since dropped to rank 30th in this hazard. About 60 percent of the particulate matter comes from industries and polluters in the neighbouring United States.

Lake Huron is cooler than the air in summer and warmer than the air in winter; therefore, it moderates Sarnia's humid continental climate, making temperature extremes of hot and cold less evident. In the winter, Sarnia occasionally experiences lake-effect snow from Arctic air blowing across the warmer waters of Lake Huron and condensing to form snow squalls over land.

The word "Sarnia" is Latin for Guernsey, a British Channel Island. In 1829 Sir John Colborne, a former governor of Guernsey, was appointed Lieutenant Governor of Upper Canada. In this capacity, he visited two small settlements in 1835 that had been laid out on the shores of Lake Huron. One of these, named "The Rapids", consisted then of 44 taxpayers, nine frame houses, four log houses, two brick dwellings, two taverns and three stores. The villagers wanted to change its name but were unable to agree on an alternative. The English settlers favoured the name "Buenos Aires", and the ethnic Scottish favoured "New Glasgow".

Sir John Colborne suggested Port Sarnia. On 4 January 1836, the name was formally adopted by a vote of 26 to 16, and Colborne also named the nearby village Moore after British military hero Sir John Moore. Sarnia adopted the nickname "The Imperial City" on 7 May 1914 because of the visit of Canada's Governor General, the Duke of Connaught, and his daughter Princess Patricia.

Ethnic French colonists, who came from Detroit, were the first European colonizers of what became Sarnia in about 1807–1810; their role is marked by a historic plaque installed by the Ontario Heritage Society. They were fur traders with the Huron and Three Fires Confederacy. At this time, the French Jesuits also established a mission near the Huron village on the river's east bank. Later, the men established farms, attracted other settlers, and stimulated growth in the area.

The township was surveyed in 1829, and in the early 1830s, a wave of Scottish immigrants settled in the area. They became dominant as English speakers and, for decades, claimed to have founded the city.