A decametre (International spelling as used by the International Bureau of Weights and Measures and by most English speaking countries, United States spelling dekameter or decameter,), symbol dam ("da" for the SI prefix deca-, "m" for the SI unit metre), is a unit of length in the International System of Units (SI) equal to ten metres.

While any combination of SI prefix and unit can be used, many are rarely used in practice; the decametre is used less frequently than other units of length. One practical use is for altitude of geopotential heights in meteorology. The volumetric form cubic decametre is convenient for describing large volumes of water such as in rivers and lakes; a volume of one cubic decametre (dam³) is equivalent to a capacity of one megalitre (ML).

One technical atmosphere is the pressure of one decameter of water. Also, the are (a), a metric unit for land area, is equal to one square decametre (dam²).