Atrial fibrillation (AF) is one of the most common cardiovascular complications in patients hospitalized with community-acquired pneumonia (CAP). However, predisposing clinical factors associated with AF in CAP patients have not been fully elucidated. We enrolled 545 patients consecutively hospitalized for CAP. Data on demographic characteristics and co-morbidities were collected and all patients underwent ECG, echocardiography, and laboratory measurements. During the in-hospital stay, 9.5% of patients experienced a new episode of AF within 24 to 72 hours from admission. CAP patients who experienced AF had a higher indexed left atrial area (LAAi) and a higher proportion of concentric left ventricular hypertrophy than those not presenting AF. Univariate logistic regression analysis showed that hypertension, history of coronary heart disease, high Pneumonia Severity Index classes, history of paroxysmal AF, systolic heart failure, concentric left ventricular hypertrophy, and an enlarged LAAi were associated with a new episode of AF. A multivariable logistic analysis showed that history of paroxysmal AF (odds ratio [OR] 11.7; 95% confidence interval [CI] 5.8 to 23.7; p <0.001), enlarged LAAi (OR 5.4; 95% CI 2.5 to 11.9; p <0.001), and concentric left ventricular hypertrophy (OR 2.2; 95 CI 1.1 to 4.6; p = 0.034) remained independently associated with AF occurrence. In conclusion, in this large cohort of CAP patients, history of paroxysmal AF, enlarged LAAi, and concentric left ventricular hypertrophy are independent predictors of AF occurrence during the early stages of pneumonia.