For decades, the source of Earth's volatiles, especially water with a deuterium-to-hydrogen ratio (D/H) of (1.558 +/- 0.001) x 10(-4), has been a subject of debate. The similarity of Earth's bulk composition to that of meteorites known as enstatite chondrites(1) suggests a dry proto-Earth(2) with subsequent delivery of volatiles(3) by local accretion(4) or impacts of asteroids or comets(5,6). Previous measurements in six comets from the Oort cloud yielded a mean D/H ratio of (2.96 +/- 0.25) x 10(-4). The D/H value in carbonaceous chondrites, (1.4 +/- 0.1) x 10(-4), together with dynamical simulations, led to models in which asteroids were the main source of Earth's water(7), with <= 10 per cent being delivered by comets. Here we report that the D/H ratio in the Jupiter-family comet 103P/Hartley 2, which originated in the Kuiper belt, is (1.61 +/- 0.24) x 10(-4). This result substantially expands the reservoir of Earth ocean-like water to include some comets, and is consistent with the emerging picture of a complex dynamical evolution of the early Solar System(8,9).