Background Interatrial block (IAB) has been associated with supraventricular arrhythmias and stroke, and even with sudden cardiac death in the general population. Whether IAB is associated with life‐threatening arrhythmias (LTA) and sudden cardiac death in dilated cardiomyopathy (DCM) remains unknown. This study aimed to determine the association between IAB and LTA in ambulant patients with DCM. Methods and Results A derivation cohort (Maastricht Dilated Cardiomyopathy Registry; N=469) and an external validation cohort (Utrecht Cardiomyopathy Cohort; N=321) were used for this study. The presence of IAB (P‐wave duration>120 milliseconds) or atrial fibrillation (AF) was determined using digital calipers by physicians blinded to the study data. In the derivation cohort, IAB and AF were present in 291 (62%) and 70 (15%) patients with DCM, respectively. LTA (defined as sudden cardiac death, justified shock from implantable cardioverter‐defibrillator or anti‐tachypacing, or hemodynamic unstable ventricular fibrillation/tachycardia) occurred in 49 patients (3 with no IAB, 35 with IAB, and 11 patients with AF, respectively; median follow‐up, 4.4 years [2.1; 7.4]). The LTA‐free survival distribution significantly differed between IAB or AF versus no IAB (both P <0.01), but not between IAB versus AF ( P =0.999). This association remained statistically significant in the multivariable model (IAB: HR, 4.8 (1.4–16.1), P =0.013; AF: HR, 6.4 (1.7–24.0), P =0.007). In the external validation cohort, the survival distribution was also significantly worse for IAB or AF versus no IAB ( P =0.037; P =0.005), but not for IAB versus AF ( P =0.836). Conclusions IAB is an easy to assess, widely applicable marker associated with LTA in DCM. IAB and AF seem to confer similar risk of LTA. Further research on IAB in DCM, and on the management of IAB in DCM is warranted.