A capillary wave is a wave traveling along the phase boundary of a fluid, whose dynamics are dominated by the effects of surface tension.

Capillary waves are common in nature and the home, and are often referred to as ripples. The wavelength of capillary waves in water is typically less than a few centimeters.

When generated by light wind in open water, a nautical name for them is "cat's paw" waves, since they may resemble paw prints. Light breezes which stir up such small ripples are also sometimes referred to as cat's paws. On the open ocean, much larger ocean surface waves (seas and swells) may result from coalescence of smaller wind-caused ripple-waves.

A gravity–capillary wave on a fluid interface is influenced by both the effects of surface tension and gravity, as well as by fluid inertia.

The dispersion relation for capillary waves is

where ω is the angular frequency, σ the surface tension, ρ the density of the heavier fluid, ρ' the density of the lighter fluid and k the wavenumber. The wavelength is Failed to parse (Missing texvc executable; please see math/README to configure.): \lambda=\frac{2 \pi}{k}.