Background: Little is known about sex differences in the prevalence, treatment, and outcome of atrial fibrillation complicating acute heart failure. Methods and Results: Among 957 patients (429 women, 528 men), included in the BIO-HF registry, 45.2% (n = 194) of the women and 45.1% (n = 238) of the men were admitted with atrial fibrillation. The primary end point was a composite of 1-year all-cause mortality and hospitalization for heart failure. Adjusted 1-year mortality and hospitalization rates were similar between sexes (women 38.5%, men 36.0%; OR for female gender: 1.1, 95% CI 0.65-1.86; P = .71. A significant interaction between female sex and age (P = .002) was observed; with worse prognosis for women <75 years (OR 7.17, 95% CI 1.79-28.66; P = .005) compared with men <75 years. No sex differences in in-hospital treatment, restoration of sinus rhythm (16.5% in women vs 14.2% in men; P = .58), or in-hospital mortality (5.7% in women vs 6.7% in men; P = .69) were observed. Conclusions: Among patients hospitalized with acute heart failure, no sex differences in the prevalence and management of atrial fibrillation were observed. In-hospital mortality and the composite of 1-year mortality and rehospitalization were not different between sexes, but a significant sex-age interaction was observed, with worse outcome in women <75 years versus men <75 years of age.