L'Assomption (French pronunciation: [lasɔ̃psjɔ̃]) is an off-island suburb of Montreal, in southwestern Quebec, Canada on the L'Assomption River. It is the seat of the Regional County Municipality of L'Assomption. It is located on the outer fringes of the Montreal urban area.

Most of the economy depends on the agricultural industries of the surrounding plains. It is also the cultural centre of the region.

In 1647, the L'Assomption Seignory was granted to Pierre Legardeur de Repentigny, named after the river already named such since the seventeenth century. Between 1640 and 1700, a settlement formed inside a large horseshoe-shaped meander of the L'Assomption River. Amerindians had already been visiting this site since ancient times and called it Outaragasipi meaning winding river, in reference to the river's course. They would drag their canoes across the peninsula as a short-cut for the meander, and therefore the settlement was first called Le Portage.

In 1717, the parish was formed, known thereafter as Saint-Pierre-du-Portage-de-l'Assomption and also as Saint-Pierre-et-Saint-Paul-du-Portage. In 1766, the village saw an influx of Acadian settlers. Between 1774 and 1888, L'Assomption was the most prosperous and important town between Montreal and Trois-Rivieres.

In 1845, the L'Assomption Municipality was established, abolished in 1847, but reestablished as a parish municipality in 1855. In 1846, the village itself became a separate Village Municipality and obtained town status in 1888.

In 1992, the town and parish municipality were merged again, and on July 1, 2000, the neighbouring Parish Municipality of Saint-Gérard-Majella was amalgamated with Ville de L'Assomption.

In December 2010, the 1,300-worker Electrolux factory announced that it would close, relocating to Memphis, Tennessee.

• Domaine-Beaudoin-Papin
• Domaine-des-Fleurs
• L'Assomption
• Saint-Gérard-Majella

In the 2021 Census of Population conducted by Statistics Canada, L'Assomption had a population of 23,442 living in 9,206 of its 9,369 total private dwellings, a change of 4.5% from its 2016 population of 22,429. With a land area of 98.74 km (38.12 sq mi), it had a population density of 237.4/km (614.9/sq mi) in 2021.

Canada Census Mother Tongue – L'Assomption, Quebec
Census		Total		French				English				French & English				Other
Year		Responses		Count	Trend	Pop %		Count	Trend	Pop %		Count	Trend	Pop %		Count	Trend	Pop %
2021		23,245		21,775	2.4%	93.7%		245	19.5%	1.1%		165	43.5%	0.7%		865	61.7%	3.7%
2016		22,220		21,270	11.0%	95.7%		205	5.1%	0.9%		115	15.0%	0.5%		535	50.7%	2.4%
2011		19,815		19,165	19.0%	96.7%		195	2.5%	1.0%		100	185.7%	0.5%		355	77.5%	1.8%
2006		16,545		16,110	7.5%	97.4%		200	8.1%	1.2%		35	22.2%	0.2%		200	53.8%	1.2%
2001		15,340		14,980	37.8%	97.7%		185	131.2%	1.2%		45	28.6%	0.3%		130	160.0%	0.9%
1996		11,035		10,870	n/a	98.5%		80	n/a	0.7%		35	n/a	0.3%		50	n/a	0.5%

This section needs expansion. You can help by adding to it. (September 2017)

The Sir Wilfrid Laurier School Board operates anglophone public schools, including:

• Joliette Elementary School in Saint-Charles-Borromée

The Commission scolaire des Affluents is the main school board in the region. It operates many francophone public schools, both at the elementary and high school levels, including:

• Paul-Arseneau High School in l'Assomption
• Armand-Corbeil High School in Terrebonne
• Jean-Baptiste Meilleur High School in Repentigny
• Point-du-Jour Elementary School in l'Assomption

The city is also home to the Collège de L'Assomption, a private High School of historical renown, as well as the Cégep Régional de Lanaudière in l'Assomption.

• List of cities in Quebec