Thorold is a city in Ontario, Canada, located on the Niagara Escarpment. It is also the seat of the Regional Municipality of Niagara. The Welland Canal passes through the city, featuring lock 7 and the Twin Flight Locks.

The first survey of Thorold, or Township 9 as it was known then, occurred in 1788. The earliest communities in what is now Thorold emerged at Beaverdams, DeCew Falls and St. Johns. In 1846, the community had a population of about 1,000 and there were three churches or chapels and a post office. Various types of tradesmen worked here. Industry included two grist mills, a cement mill, a brewery and three wagon makers. There were seven taverns. Thorold, located on the brow of the Niagara Escarpment, soon became dominant and was incorporated as a village in 1850. Its formation was linked to the creation of the First Welland Canal. The village experienced population growth as the canal became more developed. In 1875, Thorold became a town.

When the Regional Municipality of Niagara was formed in 1970, the Town of Thorold expanded to include the former Thorold Township. In 1975, the town became incorporated as the City of Thorold.

Thorold is also the location of the War of 1812 battle site, Beaverdams, where, on June 25, 1813, Colonel Charles Boerstler and his American troops were defeated by the British regulars and Caughnawaga Mohawks.

The 2021 Canadian census identified Thorold as the eighth-fastest growing municipality in Canada.

In the 2021 Census of Population conducted by Statistics Canada, Thorold had a population of 23,816 living in 9,095 of its 9,856 total private dwellings, a change of 26.7% from its 2016 population of 18,801. With a land area of 83.29 km² (32.16 sq mi), it had a population density of 285.9/km² (740.6/sq mi) in 2021.

Thorold's economy, like other municipalities in the Niagara Region, has benefitted from the Welland Canal. An estimated 37 million tonnes of freight is handled and shipped there annually.

Thorold South has several industrial and manufacturing businesses.