Chemotherapy is the standard therapy for small cell lung cancer (SCLC), but relapse is common and the 2-year survival rate remains low. Given the contribution of the tumor microenvironment (TME) to cancer development and response to treatment, we analyzed here how chemotherapy alters the TME in SCLC using single-cell RNA sequencing. The comparison between neuroendocrine cells and other epithelial cells in five chemotherapy-naive patients identified upregulation of Notch-inhibiting genes, such as DLL3 and HES6. Analysis of genes differentially expressed between five patients receiving chemotherapy and five treatment-naive patients in cells in the TME showed that chemotherapy promoted antigen presentation and senescence in neuroendocrine cells, upregulated ID1 to enhance angiogenic activities of stalk-like endothelial cells and strengthened vascular endothelial growth factor signaling in lymphatic endothelial cells. Chemotherapy also promoted the remodeling of extracellular matrix by fibroblasts and upregulated interferon-mediated antitumor immune responses by B and T cells. Our single-cell transcriptome analysis provides insights into how chemotherapy affects the TME in SCLC, which may guide efforts to make therapy more effective.© 2023 UICC.