Inflammation contributes to deep vein thrombosis (DVT) formation in patients with aSAH after endovascular treatment. The relationship between systemic immune-inflammatory index (SII) as an inflammatory marker and DVT formation remains unclear. Thus, this study aims to evaluate the association between SII and aSAH-associated DVT following endovascular treatment. We enrolled 562 consecutive patients with aSAH after endovascular treatment at three centers from January 2019 to September 2021. The endovascular treatments included simple coil embolization and stent-assisted coil embolization. Deep venous thrombosis (DVT) was assessed by Color Doppler ultrasonography (CDUS). Multivariate logistic regression analysis was used to establish the model. We assessed the association of the SII, neutrophil-to-lymphocyte ratio (NLR), the systemic inflammatory response index (SIRI), platelet-lymphocyte ratio (PLR), and DVT by using restricted cubic spline (RCS). ASAH-associated DVT was found in 136 (24.20%) patients. Based on the multiple logistic regression analysis, the correlation was found between aSAH-associated DVT and elevated SII (fourth quartile) (adjusted odds ratio = 8.20 [95% confidence interval, 3.76-17.92]; p < 0.001 [p for trend < 0.001]), elevated NLR (fourth quartile) (adjusted odds ratio = 6.94 [95% confidence interval, 3.24-14.89]; p < 0.001 [p for trend < 0.001]), elevated SIRI (fourth quartile) (adjusted odds ratio = 4.82 [95% confidence interval, 2.36-9.84]; p < 0.001 [p for trend < 0.001]), and elevated PLR (fourth quartile) (adjusted odds ratio = 5.49 [95% confidence interval, 2.61-11.57]; p < 0.001 [p for trend < 0.001]). The increased SII was correlated with the formation of aSAH-associated DVT after endovascular treatment.© 2023. The Author(s), under exclusive licence to Springer-Verlag GmbH Germany, part of Springer Nature.