Background: Biomass extract of Streptomyces sanyensis (BE-SS), which is a marine microorganism, has antitumor activity and has white-to-gray aerial mycelium and long chains of long spores in aerial mycelium. Objectives: The anti-cancer effect of BE-SS on triple-negative breast cancer (TNBC) was confirmed and cell death by BE-SS was confirmed to have immunogenicity and whether it could be used for immuno-cancer therapy. Materials and Methods: Human and mouse TNBC cells (MDA-MB-231, HS578T, 4T1-hemagglutinin (HA) and TUBO-P2J-HA cells) were treated with BE-SS at different concentrations for 72 hr and their cytotoxicity was detected using the sulforhodamine B-based (SRB) method. Flow cytometry was used to determine the type of cell death by Annexin V/7-AAD staining and to measure surface exposure to damage-related molecular pattern (DAMP) molecules including calreticulin (CRT), heat shock protein (HSP) 70/90. Carboxyfluorescein succinimidyl ester (CFSE) dilution assay was used to evaluate the immunogenicity of dead cells induced by BE-SS. Results: BE-SS reduced the viability of human (MDA-MB-231 and HS578T-cells) and mouse (4T1-HA and TUBO-P2J-HA cells) breast cancer cells. At 72 h, the IC50 of human and mouse breast cancer cells was 0.02-0.6 μg/ml and 0.005-0.37 μg/ml, respectively. BE-SStreated breast cancer cells were positively stained with Annexin V. Surface exposure to DAMP molecules increased in a dose-and time-dependent manner. CFSE dilution analysis showed that dendritic cells (DCs) fed with BE-SS treated breast cancer cells successfully stimulated tumor-specific T-cell proliferation without inhibiting DC function and T-cell proliferation. Conclusion: BE-SS can induce immunogenicity and apoptosis in breast cancer cells and may be a good adjuvant for TNBC immunotherapy.