Background Bleeding is frequent in patients with atrial fibrillation (AF) treated with oral anticoagulant therapy, and may be the first manifestation of underlying cancer. We sought to investigate to what extent bleeding represents the unmasking of an occult cancer in patients with AF treated with oral anticoagulants. Methods and Results Using data from CardioCHUVI-AF (Retrospective Observational Registry of Patients With Atrial Fibrillation From Vigo's Health Area), 8753 patients with AF aged ≥75 years with a diagnosis of AF between 2014 and 2017 were analyzed. Of them, 2171 (24.8%) experienced any clinically relevant bleeding, and 479 (5.5%) were diagnosed with cancer during a follow-up of 3 years. Among 2171 patients who experienced bleeding, 198 (9.1%) were subsequently diagnosed with cancer. Patients with bleeding have a 3-fold higher hazard of being subsequently diagnosed with new cancer compared with those without bleeding (4.7 versus 1.4 per 100 patient-years; adjusted hazard ratio HR, 3.2 95% CI, 2.6-3.9). Gastrointestinal bleeding was associated with a 13-fold higher hazard of new gastrointestinal cancer diagnosis (HR, 13.4; 95% CI, 9.1-19.8); genitourinary bleeding was associated with an 18-fold higher hazard of new genitourinary cancer diagnosis (HR, 18.1; 95% CI, 12.5-26.2); and bronchopulmonary bleeding was associated with a 15-fold higher hazard of new bronchopulmonary cancer diagnosis (HR, 15.8; 95% CI, 6.0-41.3). For other bleeding (nongastrointestinal, nongenitourinary, nonbronchopulmonary), the HR for cancer was 2.3 (95% CI, 1.5-3.6). Conclusions In patients with AF treated with oral anticoagulant therapy, any gastrointestinal, genitourinary, or bronchopulmonary bleeding was associated with higher rates of new cancer diagnosis. These bleeding events should prompt investigation for cancers at those sites.