Radial velocity is the velocity of an object in the direction of the line of sight (i.e. its speed straight towards or away from an observer). In astronomy, radial velocity most commonly refers to the spectroscopic radial velocity. The spectroscopic radial velocity is the radial component of the velocity of the source at emission and the observer at observation, as determined by spectroscopy. Astrometric radial velocity is the radial velocity as determined by astrometric observations (for example, a secular change in the annual parallax).

Light from an object with a substantial relative radial velocity at emission will be subject to the Doppler effect, so the frequency of the light decreases for objects that were receding (redshift) and increases for objects that were approaching (blueshift).

The radial velocity of a star or other luminous distant objects can be measured accurately by taking a high-resolution spectrum and comparing the measured wavelengths of known spectral lines to wavelengths from laboratory measurements. A positive radial velocity indicates the distance between the objects is or was increasing; a negative radial velocity indicates the distance between the source and observer is or was decreasing.