This study is designed to illuminate the specific role and underlying mechanism of collagen type III alpha 1 chain (COL3A1) in triple negative breast cancer (TNBC). Quantitative real-time polymerase chain reaction was applied to examine mRNA expression of COL3A1. Western blot analysis was employed to determine protein levels of COL3A1, programmed death ligand 1 (PD-L1), Bcl-2, and cleaved caspase-3. Immunohistochemistry staining was utilized for assessing protein expression of Ki67 and COL3A1 in tissues. The proliferous capacity of cells was assessed through CCK-8 assay and 5-Ethynyl-2'-deoxyuridine assay. Cell apoptosis and the percentage of CD8 T cells were measured using flow cytometry. Migration and invasion of TNBC cells were examined via transwell assay. Lactate dehydrogenase (LDH) release was measured via a LDH assay kit. For establishing a xenograft tumor model, MDA-MB-231 cells were injected into the flank of mice through subcutaneous injection. COL3A1 expression was raised in TNBC tissues and cells, and it was inversely associated with overall survival data of TNBC patients. COL3A1 downregulation repressed proliferation, invasion, migration, and immune escape of TNBC cells along with tumor growth of xenograft mice. In TNBC cells and tumor tissues of mice, protein expression of PD-L1 was reduced by COL3A1 knockdown. COL3A1 knockdown-mediated inhibitory effects on cell proliferation, migration, invasion, and immune escape were reversed by PD-L1 upregulation in vitro. Silencing of COL3A1 exerted an antitumor role in TNBC, implying its potential as a therapeutic target for TNBC.