THE well studied(1-5) Vela pulsar, which has recently(6,7) been demonstrated to lie at the centre of the Vela supernova remnants, loses far more rotational energy than can be accounted for by radiation from the pulsar itself or from a surrounding, compact nebula(9-11). One theory suggests that the energy is carried away by an equatorial wind(12). Here we report the observation of an X-ray-emitting jet, which begins at the pulsar and extends roughly seven parsecs towards the south-southwest and along the pulsar spin axis(13) (indicating a polar, rather than equatorial flow). This jet may provide the loss mechanism for most of the pulsar’s rotational spin-down energy, One-sided jets, such as Vela, may also accelerate pulsars to large space velocities.