SummaryRecent reports have raised concerns about the association of chimeric antigen receptor T cell (CAR‐T) with non‐negligible cardiotoxicity, particularly atrial arrhythmias. First, we conducted a pharmacovigilance study to assess the reporting of atrial arrhythmias following CD19‐directed CAR‐T. Subsequently, to determine the incidence, risk factors and outcomes of atrial arrhythmias post‐CAR‐T, we compiled a retrospective single‐centre cohort of non‐Hodgkin lymphoma patients. Only commercial CAR‐T products were considered. Atrial arrhythmias were nearly fourfold more likely to be reported after CAR‐T therapy compared to all other cancer patients in the FAERS (adjusted ROR = 3.76 [95% CI 2.67–5.29]). Of the 236 patients in our institutional cohort, 23 (10%) developed atrial arrhythmias post‐CAR‐T, including 12 de novo arrhythmias, with most (83%) requiring medical intervention. Atrial arrhythmias frequently co‐occurred with cytokine release syndrome and were associated with higher post‐CAR‐T infusion peak levels of IL‐10, TNF‐alpha and LDH, and lower trough levels of fibrinogen. In a multivariable analysis, risk factors for atrial arrhythmia were history of atrial arrhythmia (OR = 6.80 [2.39–19.6]) and using CAR‐T product with a CD28‐costimulatory domain (OR = 5.17 [1.72–18.6]). Atrial arrhythmias following CD19‐CAR‐T therapy are prevalent and associated with elevated inflammatory biomarkers, a history of atrial arrhythmia and the use of a CAR‐T product with a CD28 costimulatory domain.