Nearly 400 years have passed since a supernova was last observed directly in the Milky Way (by Kepler, in 1604). Numerous Galactic supernovae are expected to have occurred since then, but only one (Cassiopeia A) may have been seen. The historical record of supernovae is therefore incomplete, as demonstrated by the spatial distribution of young supernova remnants. The discovery, of γ-ray emission from the decay of 44Ti nuclei associated with Cassiopeia A, the youngest known remnant, has revealed a new way to search for the remnants of other relatively recent supernovae (less than ~1,000 years old). Here we report the discovery of 44Ti line emission from a previously unknown young supernova remnant, in the direction of the Vela remnant. We estimate a distance of ~200parsecs and an age of ~680 years for the remnant, making it the closest young remnant to the Earth. Why it was not recorded historically remains unknown.