It is located next to the Lapua River in the province of Western Finland and is part of the Southern Ostrobothnia region. The town has a population of 14,628 (31 March 2016) and covers an area of 738.15 square kilometres (285.00 sq mi) of which 13.67 km² (5.28 sq mi) is water. The population density is 19.85 inhabitants per square kilometre (51.4/sq mi). The municipality is unilingually Finnish.

The Battle of Lapua was fought between Swedish and Russian forces near the outskirts of the town on 14 July 1808 as part of the Finnish War. Lapua is the seat of the Evangelical Lutheran Diocese of Lapua. The Lapua Cathedral, designed by Carl Ludvig Engel, was built in 1827. In the 1930s the radical anti-communist Lapua Movement was founded and named after the town.

Lapua is also home to a large ammunition factory, which commenced operations in 1927 as the State Cartridge Factory. This factory was the primary supplier of ammunition to the Finnish Army during the Winter War and World War II. An explosion occurred in a warehouse of this factory on 13 April 1976, resulting in the deaths of 40 employees, mainly females. Sixty children lost a parent in the disaster. This is the worst accidental disaster in Finland's modern history. After the explosion, the factory was relocated 5 kilometres (3 mi) away from the town centre and continues production today as part of the Nordic Ammunition Group (Nammo) as Nammo Lapua. The original site of the factory and the surviving buildings are now an arts centre, a library and a theater.

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