Introduction: As one of the leading causes of cancer-related deaths in women, breast cancer still poses major treatment challenges. Chemotherapy often causes toxicity and resistance, emphasizing the need for safer options. Cassia alata, rich in flavonoids like kaempferol shows potential anticancer effects through apoptosis induction. This review evaluated the in vitro evidence of C. alata against human breast cancer cell lines. Methods: This review followed the PRISMA 2020 guidelines. A total of 1,183 records were retrieved from six databases (PubMed, Scopus, ScienceDirect, ProQuest, EBSCOhost, and Google Scholar). After screening records, eight studies met the eligibility criteria. Eligible studies were in vitro experiments evaluating C. alata crude extracts, fractions, or derived formulations on human breast cancer cell lines (MCF-7, T47D, MDA-MB-231, and others). Data on interventions, concentrations, cytotoxicity assays, and molecular mechanisms were extracted. Quality assessment was performed independently by two reviewers using the ToxRTool, with discrepancies resolved through consensus. Inter-rater reliability was not applied due to small sample size and consensus scoring. Results: C. alata consistently reduced breast cancer cell viability, with reported IC₅₀ values ranging from 0.013 to 456 µg/mL depending on extract and cell line. Most studies showed dose-dependent cytotoxicity and apoptosis via caspase activation, nuclear fragmentation, and PI3K/AKT inhibition. Variations in extraction methods and assay conditions explained inter-study heterogeneity. Conclusion: Evidence from studies indicates that C. alata reduces breast cancer cell viability and promotes apoptosis. Further in vivo and clinical studies are required to validate efficacy and safety.