Thorold, city, regional municipality of Niagara, southeastern Ontario, Canada. It lies along the Welland Canal, 4 miles (6.5 km) south of St. Catharines. Founded in 1788 and named after a British member of Parliament, Sir John Thorold, the town grew with the development of the canal, beginning in 1829. Thorold is now an industrial and shipping centre, on the edge of the Niagara Escarpment, at the Twin Flight Locks. Its industries include paper, pulp, and lumber milling, quarrying, and the manufacture of abrasives. Inc. town, 1875; city, 1975. Pop. (2006) 18,224; (2011) 17,931.