Greenhouse warming is a predicted consequence of the Chicxulub impact, but supporting data are sparse. This shortcoming compromises understanding of the impact's effects, and it has persisted due to an absence of sections that both contain suitable material for traditional carbonate-or organic-based paleothermometry and are complete and expanded enough to resolve changes on short time scales. We address the problem by analyzing the oxygen isotopic composition of fish debris, phosphatic microfossils that are relatively resistant to diagenetic alteration, from the Global Stratotype Section and Point for the Cretaceous/Paleogene boundary at El Kef, Tunisia. We report an similar to 1 per mil decrease in oxygen isotopic values (similar to 5 degrees C warming) beginning at the boundary and spanning similar to 300 centimeters of section (similar to 100,000 years). The pattern found matches expectations for impact-initiated greenhouse warming.