Purpose <p>The spleen plays a significant role in innate and adaptive immunity. We aimed to evaluate the prognostic significance of the splenic volume in patients with diffuse large B-cell lymphoma (DLBCL), research on which is limited.</p> Methods <p>We retrospectively analyzed 175 patients diagnosed with DLBCL. Based on optimal thresholds, patients were stratified into small (&lt; 185 cm³), medium (185–315 cm³), and large (≥ 315 cm³) splenic volume groups. Survival analysis was performed using Kaplan–Meier and Cox proportional hazards model.</p> Results <p>A significant decrease in progression-free survival (PFS) (<i>P</i> &lt; 0.0001) and overall survival (OS) (<i>P</i> &lt; 0.0001) was observed in the large group that received cyclophosphamide, hydroxydaunorubicin, vincristine, and prednisone (CHOP) without or with rituximab (R-CHOP) treatment. Among the 139 patients who received R-CHOP, significant adverse effects on PFS (<i>P</i> &lt; 0.0036) and OS (<i>P</i> &lt; 0.0035) were observed in the large group. Univariate and multivariate analyses demonstrated that enlarged splenic volume (≥ 315 cm³) was associated with decreased PFS (<i>P</i> &lt; 0.0001, <i>P</i> &lt; 0.01, respectively) and OS (<i>P</i> &lt; 0.001, <i>P</i> &lt; 0.01, respectively). In terms of peripheral blood inflammatory markers, patients with enlarged splenic volume (≥ 315 cm³) exhibited lymphocytopenia, increased monocyte-to-lymphocyte ratio (MLR), increased neutrophil-to-lymphocyte ratio (NLR), and higher red blood cell distribution width (RDW) compared to that in those with smaller splenic volumes (<i>P</i> &lt; 0.001, <i>P</i> &lt; 0.001, <i>P</i> = 0.01, and <i>P</i> &lt; 0.005, respectively).</p> Conclusion <p>Splenic volume exceeding 315 cm³ is a poor prognostic factor for PFS and OS in patients with DLBCL. An enlarged splenic volume may be associated with compromised anti-cancer immune responses and aberrant inflammatory conditions.</p>