Few studies have addressed to date the interaction between sex and diabetes mellitus (DM) in the prognosis of elderly patients with non-ST-segment elevation acute coronary syndrome (NSTEACS). Our aim was to address the role of DM in the prognosis of non-selected elderly patients with NSTEACS according to sex. A retrospective analysis from 11 Spanish NSTEACS registries was conducted, including patients aged ≥70 years. The primary end point was one-year all-cause mortality. A total of 7211 patients were included, 2,770 (38.4%) were women, and 39.9% had DM. Compared with the men, the women were older (79.95 ± 5.75 vs. 78.45 ± 5.43 years, p < 0.001) and more often had a history of hypertension (77% vs. 83.1%, p < 0.01). Anemia and chronic kidney disease were both more common in women. On the other hand, they less frequently had a prior history of arteriosclerotic cardiovascular disease or comorbidities such as peripheral artery disease and chronic pulmonary disease. Women showed a worse clinical profile on admission, though an invasive approach and in-hospital revascularization were both more often performed in men (p < 0.001). At a one-year follow-up, 1090 patients (15%) had died, without a difference between sexes. Male sex was an independent predictor of mortality (HR = 1.15, 95% CI 1.01 to 1.32, p = 0.035), and there was a significant interaction between sex and DM (p = 0.002). DM was strongly associated with mortality in women (HR: 1.45, 95% CI = 1.18–1.78; p < 0.001), but not in men (HR: 0.98, 95% CI = 0.84–1.14; p = 0.787). In conclusion, DM is associated with mortality in older women with NSTEACS, but not in men.