The cytotoxic effect of the tetrameric peptide LfcinB (20-25) against breast cancer cell line ATCC® HTB-22™ (MCF-7) was evaluated. The tetrameric peptide exhibited a concentration-dependent cytotoxic effect against MCF-7 cancer cells. The peptide at 22 µM had the maximum cytotoxic effect against MCF-7 cancer cells, reducing their cell viability to ∼20%. The cytotoxic effect of the tetrameric peptide against MCF-7 cells was sustained for 24 hours. Furthermore, the tetrameric peptide did not exhibit a significant cytotoxic effect against the non-tumorogenic trophoblastic cell line, which confirms their selectivity for breast cancer cell lines. The MCF-7 cells treated at 12.2 µM for 1 h exhibited morphological changes characteristic of apoptosis, such as rounded forms and cellular shrinkage. Furthermore, this peptide induces severe cellular damage to MCF-7 cells, mitochondrial membrane depolarization, and increase of cytoplasmic calcium concentration. Our results suggest that it has a significant selective cytotoxic effect against MCF-7 cells, which may be mainly associated with the apoptotic pathway. This peptide, which contains the RRWQWR motif, could be considered to be a promising candidate for developing therapeutic agents for the treatment of breast cancer.