A boundary marker, border marker, boundary stone, or border stone is a robust physical marker that identifies the start of a land boundary or the change in a boundary, especially a change in direction of a boundary. There are several other types of named border markers, known as boundary trees, pillars, monuments, obelisks, and corners. Border markers can also be markers through which a border line runs in a straight line to determine that border. They can also be the markers from which a border marker has been fixed.

According to Josiah Ober, boundary markers are "a way of imposing human, cultural, social meanings upon a once-undifferentiated natural environment." Boundary markers are linked to social hierarchies, since they derive their meaning from the authority of a person or group to declare the limits of a given space of land for political, social or religious reasons. Ober notes that "determining who can use parcels of arable land and for what purpose, has immediate and obvious economic implications."

Many borders were drawn along invisible lines of latitude or longitude, which often created a need to mark these borders on the ground, as accurately as possible, using the technology of the day. Advances in GPS technology have shown that there are many borders inaccurately marked on the ground.

Boundary markers have often been used to mark critical points^{[clarification needed]} on political boundaries, i.e. those between countries, states or local administrations, but have also been used to mark out the limits of private landholdings, especially in areas where fences or walls are impractical or unnecessary.^{[citation needed]}

Boundary markers are integral to boundary law in the United States, both in the original colonial states and those added later during westward expansion (otherwise known as the Public Land Survey System). Man-made boundary markers, or monuments, are considered to be second-highest in the Order of Evidence in boundary law in the United States, behind only natural markers such as boulders and rivers. Boundary markers also have legal meaning in Japan, and are generally installed across the country. Markers are still used extensively for marking international borders, which are traditionally classified into two categories: natural boundaries, correlating to topographical features such as rivers or mountain ranges, and artificial boundaries, which have no obvious relation to topography. The latter category includes borders defined by boundary markers such as stones and walls. International boundary markers are placed and can be maintained by mutual agreement of the bordering countries.

Boundary markers, traditionally, were often made of stone, but later many have been made with concrete or a mixture of materials. They are typically placed at a notable or especially visible point. Many are inscribed with relevant information such as the abbreviation of the boundary holder and often a date.

The oldest known boundary stone in China is from Jiangsu Province. Dating from 12 A.D., it bears the inscription "the sea area from Jiaozhou Bay to the east of Guixan county belongs to Langya Shire and the waters from the south of Guixan county to the east of the estuary of Guanhe River belongs to Donghai Shire". More recently, the border between Russia and China was formally demarcated with boundary stones as the result of the Treaty of Kiakhta in 1727. In the nineteenth century, stones were used to outline the limits of the International Settlement in Shanghai.

In ancient Thailand, sacred boundary stones called Sema Hin delimited Buddhist temple precincts. In some cases they feature inscriptions recounting the history of the temple; others were carved with wheels of the law, while some specimens consist of unfinished stone. In addition to temples, sema could enclose statues of Buddha or sacred mounds.