Abstract Introduction Resistance to immune checkpoint inhibitors (ICI) is a significant issue in metastatic renal cell carcinoma (mRCC), as it is in the majority of cancer types. An important deficiency in immunooncology today is the lack of a predictive factor to identify this patient group. Myeloid-derived suppressor cells (MDSC) are a type of cell that contributes to immunotherapy resistance by inhibiting T cell activity. While it accumulates in the tumor microenvironment and blood, it can also accumulate in lymphoid organs such as the spleen and cause splenomegaly. Therefore we aimed to evaluate the effect of increase in splenic volume, which can be considered as an indirect indicator of increased MDSC cells, on survival outcomes in mRCC patients. Methods We analyzed 45 patients with mRCC who received nivolumab as a second-line or subsequent therapy. Splenic volume was analyzed from baseline imaging before starting nivolumab and from control imaging performed within the first 6 months of treatment initiation. Additionally, we analyzed how patients’ body mass index (BMI), IMDC risk score, ECOG performance status, nephrectomy status, neutrophil-lymphocyte ratio (NLR), Platelet-to-lymphocyte ratio (PLR) and sites of metastasis. Results Median splenic volume change was 10% (ranging from − 22% to + 117%) during follow-up. Change in splenic volume was found to be associated with overall survival (OS) and progression-free survival (PFS) (p = 0.025, 0.04). The median PFS in patients with increased splenic volume was 5 months, while it was 17 months in patients without increased splenic volume. (HR 2.1, 95% CI (1–4), p = 0.04). The median OS in patients with increased splenic volume was 9 months, while it was 35 months in patients without increased splenic volume (HR 2.7, 95% CI (1.1–6.2), p = 0.025). In four patients with decreased splenic volume, neither PFS nor OS could reach the median value. Log-rank p value in respectively (0.015, 0.035), The group in which an increase in volume was accompanied by a high NLR had the shortest survival rate. Basal splenic volume was analyzed separately. However, neither PFS nor OS differed significantly. Conclusion Our findings suggest that the change in splenic volume throughout immunotherapy regimens may be utilized to predict PFS and OS in mRCC patients undergoing treatment.