Niagara-on-the-Lake is a town in Ontario, Canada. It is located on the Niagara Peninsula at the point where the Niagara River meets Lake Ontario, across the river from New York, United States. Niagara-on-the-Lake is in the Niagara Region of Ontario and is the only town in Canada that has a lord mayor. It had a population of 19,088 as of the 2021 Canadian census.

Niagara-on-the-Lake is important in the history of Canada: it served as the first capital of the province of Upper Canada, the predecessor of Ontario. It was called Newark from 1792 to 1797. During the War of 1812, the town, the two former villages of St. David's and Queenston, and Fort George were the sites of numerous battles following the American invasion of Upper Canada, and the town was razed. Niagara-on-the-Lake is home to the oldest Catholic church, the second-oldest Anglican church in Ontario, and the oldest surviving golf course in North America.

Today, Niagara-on-the-Lake draws tourists with its colonial-style buildings, the Shaw Festival, Fort George, wineries, an outlet mall on the highway, and its proximity to Niagara Falls. The Niagara Region has the second-highest percentage of seniors in Ontario.

Before the British settlers came, the point where Fort Mississauga is situated was inhabited by at least three Native American tribes: the Neutral (15th century); Seneca (late 17th century); and Mississauga (18th century).^{[citation needed]}

The settlement was founded in 1781 as Butlersburg in honour of Colonel John Butler, the commander of Butler's Rangers. It was later renamed West Niagara to distinguish it from Fort Niagara. It was a British military base and haven for pro-British loyalists fleeing the United States during the volatile aftermath of the American Revolution. Renamed Newark by Lieutenant-Governor John Graves Simcoe in 1792, it was the first capital of Upper Canada (now the province of Ontario). The Upper Canada legislature first met at Navy Hall on September 17, 1792, and met there four more times until June 1796. In 1797, Simcoe moved the capital to York because Newark was very close to the border with the U.S. Newark was renamed Niagara in 1798.

Fort George was built just south of the settlement in 1796–1799. During the War of 1812, Niagara was taken in the Battle of Fort George by American forces in May 1813 after a two-day bombardment by cannon from Fort Niagara and the American fleet, followed by a fierce battle. After capturing Fort George, the Americans built their own fortifications there. The British retook the fort in December 1813 but abandoned it in 1815. The tiny portion of the fort that still remains has been fully restored. Fort Mississauga was built starting in 1813 but was not completed until after the war in 1816. During the war, the settlement of Niagara was razed and burnt to the ground by American soldiers as they withdrew to Fort Niagara. (Afterwards, on December 19, 1813, the British captured Fort Niagara.) The citizens rebuilt Niagara after the war, with the residential quarter around Queen Street and toward King Street, where the new court house was rebuilt out of range of Fort Niagara's cannons.

The Smith's Canadian Gazetteer of 1846 describes "Niagara (formerly called Newark)" as follows:

It has been a place of considerable trade. On the east side of the town is a large military reserve. About half a mile up the river are the ruins of Fort George, where the remains of General Brock were originally interred; they were removed. A new town-hall and court-house are intended to be erected by the town. There is a fire brigade with two engines and a hook and ladder company. Churches and chapels total five. Two newspapers are published weekly ... Steamboats run daily, as long as the weather will allow of it, from Toronto ... The Niagara Harbour and Dock Company were incorporated in the year 1830 ... the vessels turned out by the Company [include] the steamboat "London," which commenced running in the spring of 1845, the fastest boat on the upper lakes ... The Company usually employ about 150 hands; and, when particularly busy, have employed as many as 350. There is also on the premises a marine railway, large enough for hauling up vessels of the first class. Post Office, post every day. Professions and Trades.—Three physicians and surgeons, nine lawyers, twelve stores, taverns, two chemists and druggists, three booksellers and stationers, two saddlers, four wagon makers, two watchmakers, two tallow-chandlers, marble works, two printers, two cabinet makers, one hatter, four bakers, two livery stables, two tinsmiths, three blacksmiths, six tailors, seven shoemakers, one tobacconist, one bank agency, ... large quantities of apples, peaches, and cider are shipped annually.