Summary Introduction Management of atrial fibrillation (AF) is hampered by frequent recurrences after restoration of sinus rhythm. Delayed interatrial conduction has been associated with the development of AF in different clinical settings. The aim of our study was to assess whether advanced interatrial block (aIAB) was associated with AF recurrence after pharmacological cardioversion with two different antiarrhythmic drugs. Methods We included 61 patients with recent onset AF without structural heart disease that underwent successful pharmacological cardioversion. Thirty‐one patients received a single oral dose of propafenone, and 30 patients received iv vernakalant. A 12‐lead ECG (filter 150 Hz, 25 mm/s, 10 mm/mV) after conversion was evaluated for the presence of interatrial block (IAB); partial (pIAB): P‐wave duration > 120 ms, and advanced (aIAB): P‐wave > 120 ms and biphasic morphology (±) in inferior leads. Clinical follow‐up and electrocardiographic recordings were performed for a 12‐month period. Results Age was 58 ± 10.4 years and 50.8% were male. aIAB was present in 11 patients (18%) and pIAB in 10 (16.4%). At 1‐year follow‐up, 22 patients (36%) had AF recurrence. The recurrence rate with aIAB was 90.9% versus 70% in those with pIAB and 12.5% in normal P‐wave duration (P = 0.001). The presence of aIAB was strongly associated with AF recurrence (odds ratio 18.4 in multivariable modeling). Recurrence was not affected by the drug used for cardioversion (P = 0.92). Conclusion Advanced interatrial block is associated with higher risk of AF recurrence at 1 year after pharmacological cardioversion, independent of the drug used.