Background: The diagnosis and management of atrial fibrillation (AF) in emergency departments (EDs) have not been well described in France, with limited EU research. This study aimed to describe the diagnosis, management, and prognosis of AF patients in French EDs. Methods: A prospective, observational 2-month study in adults diagnosed with AF was conducted at 32 French EDs. Data regarding patient characteristics, diagnosis, and treatment at EDs were collected, with 12-month follow-up. Results: The study included a total of 1369 patients diagnosed with AF at an ED: 279 patients (20.4%) with idiopathic AF (no identified cause of the AF) and 1090 (79.6%) with secondary AF (with a principal diagnosis identified as the cause of AF). Patients were aged 84 years (median) and 51.3% were female. Significantly more idiopathic AF patients than secondary AF patients underwent CHA2DS2-VASc assessment (67.8% vs. 52.1%,) or echocardiography (21.2% vs. 8.3%), or received an oral anticoagulant and/or antiarrhythmic (62.0% vs. 12.9%). Idiopathic AF patients also had significantly higher rates of discharge to home (36.4% vs. 20.4%) and 3-month cardiologist follow-up (67.0% vs. 41.1%). At 12 months, 96% of patients with follow-up achieved sinus rhythm. The estimated Kaplan-Meier 12-month mortality rate was significantly lower with idiopathic AF than secondary AF (11.9% vs. 34.5%). Conclusions: Patients diagnosed with idiopathic or secondary AF at the ED presented heterogeneous characteristics and prognoses, with those with secondary AF having worse outcomes. Further studies are warranted to optimize patients' initial evaluation in EDs and provide appropriate follow-up.