The 200-km-diameter Chicxulub structure^1–3 in northern Yucatan, Mexico has emerged as the prime candidate for the Cretaceous/Tertiary (K/T) boundary impact crater^3–6. Concentric geophysical anomalies associated with enigmatic occurrences of Upper Cretaceous breccias and andesitic rocks led Penfield and Camargo¹ to suspect that this structure was a buried impact basin. More recently, the discovery of shocked quartz grains in a Chicxulub breccia³, and chemical similarities between Chicxulub rocks and K/T tektite-like glasses^3–6 have been advanced as evidence that the Chicxulub structure is a K/T impact site. Here we present evidence from core samples that Chicxulub is indeed a K/T source crater, and can apparently account for all the evidence of impact distributed globally at the K/T boundary without the need for simultaneous multiple impacts or comet showers. Shocked breccia clasts found in the cores are similar to shocked lithic fragments found worldwide in the K/T boundary ejecta layer^7,8. The Chicxulub melt rocks that we studied contain anomalously high levels of iridium (up to 13.5 parts per 10⁹), also consistent with the iridium-enriched K/T boundary layer⁹. Our best estimate of the crystallization age of these melt rocks, as determined by ⁴⁰Ar/<³⁹Ar analyses, is 65.2 ±0.4 (1σ) Myr, in good agreement with the mean plateau age of 64.98 ± 0.05 Myr recently reported¹⁰. Furthermore, these melt rocks acquired a remanent magnetization indicating that they cooled during an episode of reversed geomagnetic polarity. The only such episode consistent with⁴⁰Ar/<³⁹Ar constraints is chron 29R, which includes the K/T boundary.