Background: Delta-like ligand 3 (DLL3) frequently expressed in small cell lung cancer (SCLC) and has been emerged as a therapeutic target. However, it remains unclear how DLL3 expression status affects tumor microenvironment (TME)-mediated immune profiles and clinical outcomes of platinum doublet chemotherapy plus ICIs in SCLC patients. Methods: We retrospectively reviewed post-surgical limited-stage (LS)-SCLC, and extensive-stage (ES)-SCLC patients treated with platinum and etoposide (PE) plus anti-PD-L1 antibody. In LS-SCLC cohort, the transcriptome and whole-exome sequencing (WES) analysis were performed to assess the immune profiles, mutation status and neoantigen status. In ES-SCLC cohort, the association between the efficacy of immunochemotherapy and DLL3 expression by transcriptome analysis were evaluated. Results: Fifty-nine and 17 patients were included in the LS- and ES-SCLC cohort, respectively. In LS-SCLC cohort (n = 59), WES analysis revealed that SCLC with DLL3_High had a significantly higher number of neoantigens (77 [95% confidence interval [CI] 65-113 ] vs. 48 [95% CI 17-58], p = 0.004), and a significantly higher rate of signature SBS4 associated with smoking (43% [95% CI 28-53] vs. 28% [95% CI 15-36], p = 0.02), although TMB did not differ according to DLL expression (6.6 mut/Mb [95% CI 5.3-9.6] vs. 6.5 mut/Mb [95% CI 5.2-9.9], p = 0.26). The transcriptome analysis in LS-SCLC cohort revealed that SCLC with DLL3_High tumors had significantly suppressed immune-related pathways (IFN-γ response, inflammatory response, and TNFα signaling via nfkb), and dendritic cells (DC) function (DC Differentiation, DC Antigen Processing and Presentation, DC Migration). Profiling tumor infiltrating immune cells with CIBERSORT showed that SCLC with DLL3_High had significantly lower proportions of T-cells, macrophages, and DCs compared with those with DLL3_low. These results suggested that DLL3_High tumors suppressed tumor immunity by inhibiting antigen-presenting function. In ES-SCLC cohort (n = 17), the PFS of PE plus anti-PD-L1 antibody in patients with DLL3_High was significantly worse than those in patients with DLL3_Low (4.1 months [95% CI, 2.0-6.3] vs. 7.4 months [95% CI, 2.5-12.3]; HR 3.78 [95% CI, 1.1-12.5], p = 0.03). Conclusions: SCLC with high DLL3 expression was characterized by resistance to immunochemotherapy due to suppressed tumor immunity, although those tumors had higher neoantigen loads.