Background: We aimed to investigate the relationship between the radiation dose received by the spleen and the hematological parameters of patients who underwent chemoradiotherapy after the diagnosis of locally advanced pancreatic cancer (LAPC).Materials and Methods: Patients with LAPC who were treated with chemoradiotherapy were retrospectively reviewed. Surgical status,chemoradiotherapy details, complete blood count values (baseline, mid-treatment, and end-of-treatment), mean spleen dose, and dose-volume parameters (V5, V10, V15, V20, V25, and V30) were recorded. The relationship between spleen dose-volume parameters and the development of grade 3 lymphopenia was evaluated by Spearman's rank correlation and receiver operating characteristic (ROC) analysis. Results: All dose parameters for the spleen were significantly correlated with the mid-treatment absolute lymphocyte count. In the ROC analysis, mean spleen dose (p=0.011; area under the curve [AUC]: 0.856; 95% confidence interval [CI]: 0.675-0.995), V15 (p=0.020; AUC: 0.938; 95% CI: 0.830-0.997), and V20 (p=0.002; AUC: 0.940; 95% CI: 0.811-0.1000) were significantly associated with mid-treatment grade ≥3 lymphopenia. Conclusion: A significant correlation was found between the dose received by the spleen during chemoradiotherapy in LAPC patients and the development of lymphopenia. Contouring the spleen as an organ at risk (OAR) and documenting doses is important to establish dose limitations.