OBJECTIVE Breast cancer is the most common type of cancer in women. Despite various pharmacological developments, the identification of new therapies is still required for treating breast cancer. Crab is often recommended as a traditional medicine for cancer. This study aimed to determine the in vitro effect of a hydroalcoholic crab shell extract on a breast cancer cell line. METHODS In this experimental study, MCF7 breast cancer cell line was used. Crab shell was powdered and a hydroalcoholic (70° ethanol) extract was prepared. Five concentrations (100, 200, 400, 800, and 1,000 µg/mL) were added to the cells for three periods, 24, 48, and 72 hours. The viability of the cells were evaluated using trypan blue and 3-(4, 5-dimethylthiazol-2-yl)-2,5-diphenyltetrazolium bromide assays. Cell apoptosis was determined using the terminal deoxynucleotidyl transferase dUTP nick end labeling method. Nitric oxide (NO) level was assessed using the Griess method. Data were analyzed using analysis of variance, and p<0.05 was considered significant. RESULTS Cell viability decreased depending on dose and time, and was significantly different in the groups that were treated with 400, 800, and 1,000 µg/mL doses compared to that in the control group (p<0.001). Increasing the dose significantly increased apoptosis (p<0.001). NO secretion from MCF7 cells significantly decreased in response to different concentrations of the extract in a dose- and time-dependent manner (p<0.050). CONCLUSIONS The crab shell extract inhibited the proliferation of MCF7 cells by increasing apoptosis and decreasing NO production.