Background Antithrombotic agents to treat patients with acute myocardial infarction can cause bleeding, which may reveal undiagnosed cancer. However, the relationship between bleeding and new cancer diagnosis and the prognostic impact is still unclear. Methods and Results We analyzed the new cancer diagnosis, Bleeding Academic Research Consortium 2, 3, or 5 bleeding, and all‐cause death of 10 364 patients with acute myocardial infarction without a history of previous cancer in a multicenter acute myocardial infarction registry. During a median of 4.9 years, 1109 patients (10.7%) experienced Bleeding Academic Research Consortium 2, 3, or 5 bleeding, and 338 patients (3.3%) were newly diagnosed with cancer. Bleeding Academic Research Consortium 2, 3, or 5 bleeding was associated with an increased risk of new cancer diagnosis (subdistribution hazard ratio [sHR] 3.29 [95% CI, 2.50–4.32]). In particular, there were robust associations between gastrointestinal bleeding and new gastrointestinal cancer diagnosis (sHR, 19.96 [95% CI, 11.30–29.94]) and between genitourinary bleeding and new genitourinary cancer diagnosis (sHR, 28.95 [95% CI, 14.69–57.07]). The risk of all‐cause death was not lower in patients diagnosed with new gastrointestinal cancer after gastrointestinal bleeding (hazard ratio [HR], 4.05 [95% CI, 2.04–8.02]) and diagnosed with new genitourinary cancer after genitourinary bleeding (HR, 2.79 [95% CI, 0.81–9.56]) than in patients newly diagnosed with cancer without previous bleeding. Conclusions Clinically significant bleeding, especially gastrointestinal and genitourinary bleeding, in patients with AMI was associated with an increased risk of new cancer diagnoses. However, the bleeding preceding new cancer detection was not associated with better survival. Registration URL: https://www.clinicaltrials.gov ; Unique identifier: NCT02385682 and NCT02806102.