Triple-negative breast cancer is an aggressive subtype of breast cancer characterised by the absence of oestrogen receptor, progesterone receptor, and human epidermal growth factor receptor 2 expression, a phenotype that severely limits targeted treatment options and is associated with poor clinical outcomes. Curcumin, a polyphenolic compound derived from Curcuma longa, has demonstrated broad anticancer properties in preclinical models; however, its cytotoxic activity specifically against triple-negative breast cancer cells (MDA-MB-231) using standardised in vitro methods remains incompletely characterised. This study evaluated the cytotoxic and anti-proliferative effects of curcumin on MDA-MB-231 cells using a cell viability assay and a cell staining assay, with cisplatin included as a validated positive control. Cells were treated with two-fold serial dilutions of curcumin and cisplatin (0.39–100 micromolar) for 72 hours in the viability assay, and with five fixed concentrations of curcumin (3.75, 7.5, 15, 30 and 60 micromolar) for 72 hours in the staining assay. Curcumin produced a concentration-dependent reduction in cell viability, yielding a half-maximal inhibitory concentration of 27.162 ± 11.0765 micromolar. Cisplatin produced a half-maximal inhibitory concentration of 20.267 ± 26.0745 micromolar, confirming assay validity despite greater variability associated with the positive control. In the staining assay, two independent experiments revealed a threshold, rather than a smoothly progressive, concentration-response relationship: reductions in adherent cell survival at 3.75, 7.5, and 15 micromolar were generally modest and largely non-significant relative to untreated control cells (ranging between approximately 86% and 113% survival), whereas treatment at 30 and 60 micromolar produced marked and highly statistically significant reductions in survival in both independent experiments (falling to between approximately 14% and 45% of control values; p < 0.0001). Overall treatment effects were highly significant by one-way analysis of variance in both independent experiments (Experiment 1: F = 92.17, p < 0.0001; Experiment 2: F = 251.2, p < 0.0001). These findings provide robust in vitro evidence that curcumin exerts meaningful, concentration-dependent cytotoxic and anti-proliferative activity against MDA-MB-231 triple-negative breast cancer cells, with an apparent transition from a largely sub-threshold effect to pronounced cytotoxicity occurring between 15 and 30 micromolar, supporting its further evaluation as a candidate anticancer agent.