A nanometre (American spelling: nanometer; symbol nm) is a unit of length in the metric system, equal to one billionth of a metre. The name combines the SI prefix nano- (from the Ancient Greek νάνος, nanos, "dwarf") with the parent unit name metre (from Greek μέτρον, metrοn, "unit of measurement").

The nanometre is often used to express dimensions on the atomic scale: the diameter of a helium atom, for example, is about 0.1 nm, and that of a ribosome is about 20 nm. The nanometre is commonly used to specify the wavelength of electromagnetic radiation near the visible part of the spectrum: visible light, in particular, ranges from 400 to 700 nm. In these uses, the nanometre appears to be supplanting the other common unit for atomic scale dimensions, the angstrom, which is equal to 0.1 nanometre.

This unit is often associated with the field of nanotechnology. Since late 1980s, it has also been used to describe generations of the manufacturing technology in the semiconductor industry.

The nanometre was formerly known as the millimicron, since it is 1/1000 of a micron (micrometre), and was often denoted by the symbol mµ or (more rarely) µµ.