Background Atrial dyssynchrony, but not atrial enlargement/dysfunction, reflects acute atrial histopathological changes. It has been shown to be associated with new-onset atrial fibrillation (NOAF) in various clinical conditions but was not studied in the acute phase of ST-elevation myocardial infarction (STEMI) which is the aim of the current study. Methods A total of 440 STEMI patients underwent primary percutaneous coronary intervention (PCI) and were monitored for NOAF during hospitalization. Immediately after primary PCI, P-wave dispersion was calculated and conventional/tissue Doppler echocardiography was done. Results During a median hospitalization period of 3 days, 80 (18.2%) patients developed NOAF. The group with NOAF showed significantly higher prevalence of hypertension (P = .049), higher P-wave dispersion (P = .018), higher post-PCI-corrected TIMI frame count (P < .001), and lower incidence of post-PCI myocardial blush grade 2-3 (P = .031). Indexed left atrial maximum volume (LAVI(max)), left atrial dyssynchrony, and inter-atrial dyssynchrony were significantly higher in NOAF group (P < .001, each). Using ROC curve analysis, inter-atrial dyssynchrony showed the highest diagnostic performance (AUC 85%, 95% CI: 0.77-0.94, P < .001). A cutoff value at 23.8 ms showed a good validity for predicting NOAF with a sensitivity of 93.8% and a specificity of 68.1%. Using binary logistic regression analysis, history of hypertension (OR = 10.72, P = .03), LAVI(max) (OR = 7.47, P = .04), and inter-atrial dyssynchrony (OR = 45.58, P = .001) were independent determinants of NOAF. Conclusions In the acute phase after STEMI, history of hypertension, LAVI(max,) and inter-atrial dyssynchrony were independent determinants of inhospital NOAF, with the latter being the strongest.