Background: New-onset atrial fibrillation (NOAF) complicates acute myocardial infarction (AMI) and worsens prognosis. Systemic inflammation may contribute, but the value of admission inflammatory biomarkers is uncertain.Methods: Following PRISMA 2020 (PROSPERO CRD420261470060), we searched six databases from inception to 26 September 2026 for observational studies comparing admission neutrophil-to-lymphocyte ratio (NLR), platelet-to-lymphocyte ratio (PLR), C-reactive protein (CRP), or high-sensitivity CRP (hs-CRP) between AMI patients with and without NOAF. Risk of bias was assessed with QUIPS and certainty of evidence with GRADE. We used random-effects models, prediction intervals, leave-one-out sensitivity analyses, and Doi plots with the LFK index.Results: Sixteen studies (12,509 patients; 1,103 with NOAF) were included. NOAF was associated with higher NLR (MD 1.78, 95% CI 0.44 to 3.11; I² 70.6%), CRP (MD 4.17, 1.42 to 6.91; I² 53.8%), and hs-CRP (SMD 0.84, 0.47 to 1.20; I² 88.6%). PLR did not differ significantly (three studies; MD 57.19, 95% CI −13.38 to 127.77). CRP and hs-CRP remained significant in every leave-one-out analysis, whereas NLR lost significance after omitting one study (p = 0.055). All prediction intervals crossed no effect. Funnel plots, Doi plots, and LFK indices (2.46 to 7.25) indicated asymmetry. Certainty of evidence was very low for all biomarkers.Conclusion: Higher admission NLR, CRP, and hs-CRP are associated with NOAF after AMI, but the evidence is very low certainty and PLR findings are inconclusive. These markers cannot yet identify individual high-risk patients; prospective studies with standardized sampling, continuous rhythm monitoring, and adjusted analyses are needed.