Purpose <p>To investigate whether spleen volume and its changes on imaging could serve as novel, noninvasive biomarker and predictor of renal graft function within the first post-transplant year, and to compare their prognostic performance with conventional imaging parameters such as the renal artery resistive index (RI) for predicting graft function decline.</p> Material and method <p>This retrospective study included 76 renal transplant recipients who had preoperative and 6-month postoperative non-contrast abdominal CT and renal Doppler ultrasonography. Spleen and graft volumes were measured volumetrically on CT, while RI was recorded at 1 and 6 months postoperatively. Changes in RI and spleen/graft volumes were calculated. Glomerular filtration rate at months 1 and 12 was used to determine graft function decline, defined as a ≥ 10 mL/min/1.73&#xa0;m² GFR decrease. Associations between these parameters were analyzed using non-parametric statistics.</p> Results <p>Patients with graft function decline exhibited higher preoperative (<i>p</i> = 0.020) and 6-month RI (<i>p</i> = 0.030) values and greater RI changes (<i>p</i> = 0.010). Also, these patients had smaller preoperative spleen volumes (<i>p</i> = 0.021). Patients with decreasing follow-up spleen volume at months 6 had poorer graft function at months 12 (<i>p</i> = 0.004). Graft volume and its change showed no significant association with graft function.</p> Conclusion <p>Spleen volume changes could reflect heightened systemic immune activation and limited immunologic reserve for renal transplant patients. This highlights the underrecognized bridging radiologic and immunologic role of the spleen as imaging biomarker into post-transplant immunodynamics. Routine CT spleen volumetry could offer a novel, easily accessible predictor of renal allograft function decline within first year—redefining the role of spleen in immune monitoring after renal transplantation.</p>