Spirulina, a small blue-green algae that lives in warm, alkaline seas, is one of the world's oldest life forms. With century-old history, this nutrient-rich organism has received a lot of interest in recent years for its possible health benefits and sustainable production practices. Studies have indicated its antioxidant, anti-cancer, and anti-inflammatory qualities are contributed by its bioactive compounds, mainly phycocyanin. Other compounds or elements include phycocyanobilin, carotenoids, unsaturated fatty acids, selenium, and amino acids. Breast cancer is one of the major causes of cancer related death in women. As some of the cancer therapies are highly associated with side effects, nutraceuticals such as Spirulina could be an alternative therapeutic intervention. Although some research has been conducted to investigate the anti-breast cancer effects of Spirulina in the past, its mechanism of action is not entirely understood. Previous studies suggest that it may trigger apoptosis and autophagy via modifying signalling pathways such as NF-κB and Bcl-2/Bax ratio. It also has anti-proliferative properties, inhibits cell migration, causes cell cycle arrest, and lowers cancer cells' capacity to form colonies. Phycocyanin has been shown to produce reactive oxygen species (ROS) and down-regulate anti-apoptotic molecules, which contribute to its anticancer capabilities. These findings show that Spirulina fights cancer by targeting a variety of cellular mechanisms involved in carcinogenesis. While these data imply potential benefits, more clinical trials and research into the exact mechanisms of Spirulina in cancer treatments are needed to fully demonstrate its efficacy in treating breast cancer in humans.