AbstractWe aimed to analyze the influence of atrial fibrillation (AF) prior to hospital admission (“prevalent”) and AF diagnosed during hospital admission (“incident”) on in-hospital mortality (IHM) in women and men admitted for community-acquired pneumonia (CAP) in Spain (2016–2019). We used the Spanish Register of Specialized Care‐Basic Minimum Database. We analyzed 519,750 cases of CAP in people ≥ 18 years (213,631 women (41.1%)), out of which people with prevalent AF represented 23.75% (N = 123,440), whereas people with incident AF constituted 0.60% (N = 3154). Versus no AF, crude IHM was significantly higher for prevalent AF (15.24% vs. 11.40%,p < 0.001) and for incident AF (23.84% vs. 12.24%,p < 0.001). After propensity score marching, IHM in women and men with prevalent AF neared IHM in women and men with no AF (15.72% vs. 15.52%,p = 0.425; and 14.90% vs. 14.99%,p = 0.631, respectively), but IHM in women and men with incident AF was higher than IHM in women and men with no AF (24.37% vs. 13.36%,p < 0.001; and 23.94% vs. 14.04%,p < 0.001, respectively). Male sex was associated with a higher IHM in people with prevalent AF (OR 1.06; 95% CI 1.02–1–10), but not in people with incident AF (OR 0.93; 95% CI 0.77–1–13). AF diagnosed during hospital admission was associated with a higher IHM, irrespectively of sex.