The St. Lawrence Iroquoians were an Iroquoian Indigenous people who existed until about the late 16th century. They concentrated along the shores of the St. Lawrence River in present-day Quebec and Ontario, Canada, and in the American states of New York and northernmost Vermont. They spoke the Laurentian language, a branch of the Iroquoian family.

The Pointe-à-Callière Museum estimated their numbers as 120,000 people in 25 nations occupying an area of 230,000 square kilometres (89,000 sq mi). However, many scholars believe that this estimate of the number of St. Lawrence Iroquoians and the area they controlled is too expansive. The current archaeological evidence indicates that the largest known village had a population of about 1,000 and their total population was 8,000–10,000. The traditional view is that they disappeared because of late 16th-century warfare by the Mohawk nation of the Haudenosaunee or Iroquois League, which wanted to control trade with Europeans in the valley.

Knowledge about the St. Lawrence Iroquoians has been constructed from the studies of surviving oral accounts of the historical past from the current Native people, writings of the French explorer Jacques Cartier, earlier histories, and anthropologists' and other scholars' work with archaeological and linguistic studies since the 1950s.

Archaeological evidence has established that the St. Lawrence Iroquoians were a people distinct from the other regional Iroquoian peoples, the Five Nations of the Haudenosaunee and the Wendat (Huron). However, recent archaeological finds suggest distinctly separate groups may have existed among the St. Lawrence Iroquoians as well. The name "St Lawrence Iroquoians" refers to a geographic area in which the inhabitants shared some cultural traits, including a common language, but were not politically united.

The name of the country of Canada is probably derived from the Iroquoian word kanata, which means village or settlement.

For years historians, archeologists and related scholars debated the identity of the Iroquoian cultural group in the St. Lawrence valley which Jacques Cartier and his crew recorded encountering in 1535–36 at the villages of Stadacona and Hochelaga. An increasing amount of archaeological evidence collected since the 1950s has settled some of the debate. Since the 1950s, anthropologists and some historians have used definitive linguistic and archaeological studies to reach consensus that the St. Lawrence Iroquoians were peoples distinct from nations of the Haudenosaunee Confederacy or the Wendat. Since the 1990s, they have concluded that there may have been as many as 25 tribes among the St. Lawrence Iroquoians, who numbered anywhere from 8,000 to 10,000 people. They lived in the river lowlands and east of the Great Lakes, including in present-day northern New York and Vermont.

Before this, some scholars argued that the people were the ancestors or direct relations of historic Iroquoian groups in the greater region, such as the Wendat or Mohawk, Onondaga or Oneida of the Five Nations of the Haudenosaunee encountered by later explorer Samuel de Champlain. Since the 18th century, several theories have been proposed for the identity of the St. Lawrence River peoples. The issue is important not only for historical understanding but because of Haudenosaunee and other indigenous land claims.

In 1998, James F. Pendergast, a Canadian archeologist, summarized the four major theories with an overview of evidence: