To examine the incidence and risk factors for de novo atrial fibrillation (AF) after allogeneic hematopoietic cell transplantation (HCT) and to describe the impact of AF on HCT-related outcomes.A retrospective cohort study design was used to examine AF and associated outcomes in 487 patients who underwent allogeneic HCT from 2014 to 2016 and to characterize patient- and HCT-related risk factors. A nested case-control study design was used to describe the association between pre-HCT echocardiographic measures and future AF events.The median age at HCT was 52.4 years (18.1-78.6); the median time to AF was 117.5 days (4.0-1,405.0). The 5-year cumulative incidence of AF was 10.6%. Older (≥ 50 years) age (hazard ratio [HR], 2.76; 95% CI, 1.37 to 5.58), HLA-unrelated donor (HR, 2.20; 95% CI, 1.18 to 4.12), dyslipidemia (HR, 2.40; 95% CI, 1.23 to 4.68), and pre-HCT prolonged QTc interval (HR, 2.55; 95% CI, 1.38 to 4.72) were independent risk factors for AF. Despite having comparable left ventricular systolic function, patients who developed AF were significantly more likely to have lower left atrial ejection fraction, left atrial reservoir function, and elevated tricuspid regurgitant jet velocity prior to HCT, compared with patients who did not. The incidence rate of stroke after AF was 143 per 1,000 person-years. In adjusted analyses, AF was associated with a 12.8-fold (HR, 12.76; 95% CI, 8.76 to 18.57) risk of all-cause mortality and 15.8-fold (HR, 15.78; 95% CI, 8.70 to 28.62) risk of nonrelapse mortality.The burden of AF after allogeneic HCT population is substantial, and the development of AF is associated with poor survival. We identified important associations between patient demographics, pre-HCT cardiac parameters, HCT-related exposures, and risk of AF, setting the stage for targeted prevention strategies during and after HCT.