Abstract Background The optimal therapeutic window to start intravenous immunoglobulin (IVIG) for Kawasaki disease (KD) is highly debatable. We aimed to summarize the existing literature to evaluate the therapeutic window of IVIG treatment and its correlation with clinical outcomes in KD patients. Methods We searched the databases from inception to August 26, 2022, without language restrictions. The primary outcomes were initial IVIG resistance and coronary artery lesions (CALs) in acute phase. Secondary outcome was CALs during 1–2 months of follow-up. Results 27 studies involving 41,139 patients were included in this study. Very low-quality evidence showed that the earlier IVIG treatment within 4 days had a higher IVIG-resistance rate (RR, 1.80; 95% CI, 1.50–2.15; P