Genetically predicted increases in IgD‐CD38^dim B-cell frequency were associated with a higher risk of AF (odds ratio = 1.049, 95% confidence interval 1.024–1.075, P = .0001), whereas elevated CD25 expression on IgD+CD38^dim B cells conferred a protective effect (odds ratio = 0.974, 95% confidence interval 0.962–0.986, P = 1.87 × 10‐5). Here, CD38 is linked to atrial fibrillation.