Triple-negative breast cancer (TNBC) is an aggressive phenotype of breast cancer with poor prognosis. Immunotherapy, including anti-programmed cell death protein 1 (anti-PD-1) therapy, has shown promise in treating TNBC. This study investigates the impact of regulator of G protein signaling 1 (RGS1) on the efficacy of anti-PD-1 therapy in TNBC patients. Bioinformatics analyses were conducted to analyze differentially expressed genes in tissue-resident macrophages. Functional assays, including Transwell migration, co-culture experiments, quantitative polymerase chain reaction, and cytokine production, were conducted to evaluate the impact of RGS1 on macrophage function and CD8+ T cell activity. Chromatin immunoprecipitation and luciferase assays were utilized to determine the regulatory mechanisms of RGS1 expression. RGS1 was upregulated in TNBC patients and specifically in tumor-associated macrophages (TAMs). RGS1 knockdown in M2 macrophages reduced their chemotactic migration when co-cultured with cancer cells. In vivo, RGS1 knockdown in mice sensitized tumors to anti-PD-1 therapy, leading to reduced tumor growth and metastasis, elevated CD8+ T cell infiltration, and prolonged survival. cAMP responsive element binding protein 1 (CREB1) and p300 were identified as key regulators of RGS1 expression, and their inhibition impaired M2 macrophage function, enhancing CD8+ T cell activity. The effects of CREB1/p300 blockade were negated upon RGS1 overexpression. In conclusion, this study suggests that RGS1 is critical in maintaining the M2 phenotype of macrophage and reducing the efficacy of anti-PD-1 therapy in TNBC. Targeting RGS1 in TAMs may refine the efficacy of immune checkpoint blockade and improve clinical outcomes for TNBC patients. The regulatory mechanisms involving CREB1 and p300 offer potential therapeutic targets for modulating RGS1 expression and TAM function.Copyright © 2025. Published by Elsevier Inc.