Background Potassium disturbances per se increase the risk of ventricular fibrillation (VF). Whether potassium disturbances in the acute phase of ST-segment-elevation myocardial infarction (STEMI) are associated with VF before primary percutaneous coronary intervention (PPCI) is uncertain. Methods and Results All consecutive STEMI patients were identified in the Eastern Danish Heart Registry from 1999 to 2016. Comorbidities and medication use were assessed from Danish nationwide registries. Potassium levels were collected immediately before PPCI start. Multivariate logistic models were performed to determine the association between potassium and VF. The main analysis included 8624 STEMI patients of whom 822 (9.5%) had VF before PPCI. Compared with 6693 (77.6%) patients with normokalemia (3.5-5.0 mmol/L), 1797 (20.8%) patients with hypokalemia (<3.5 mmol/L) were often women with fewer comorbidities, whereas 134 (1.6%) patients with hyperkalemia (>5.0 mmol/L) were older with more comorbidities. After adjustment, patients with hypokalemia and hyperkalemia had a higher risk of VF before PPCI (odds ratio 1.90, 95% CI 1.57-2.30, P<0.001) and (odds ratio 3.36, 95% CI 1.95-5.77, P<0.001) compared with normokalemia, respectively. Since the association may reflect a post-resuscitation phenomenon, a sensitivity analysis was performed including 7929 STEMI patients without VF before PPCI of whom 127 (1.6%) had VF during PPCI. Compared with normokalemia, patients with hypokalemia had a significant association with VF during PPCI (odds ratio 1.68, 95% CI 1.01-2.77, P=0.045) after adjustment. Conclusions Hypokalemia and hyperkalemia are associated with increased risk of VF before PPCI during STEMI. For hypokalemia, the association may be independent of the measurement of potassium before or after VF.