Biomedical alloys are paramount materials in biomedical applications, particularly in crafting biological artificial replacements. In traditional biomedical alloys, a significant challenge is simultaneously achieving an ultra‐low Young's modulus, excellent biocompatibility, and acceptable ductility. A multi‐component body‐centered cubic (BCC) biomedical high‐entropy alloy (Bio‐HEA), which is composed of non‐toxic elements, is noteworthy for its outstanding biocompatibility and compositional tuning capabilities. Nevertheless, the aforementioned challenges still remain. Here, a method to achieve a single phase with the lowest Young's modulus among the constituent phases by precisely tuning the stability of the BCC phase in the Bio‐HEA, is proposed. The subtle tuning of the BCC phase stability also enables the induction of stress‐induced martensite transformation with extremely low trigger stress. The transformation‐induced plasticity and work hardening capacity are achieved via the stress‐induced martensite transformation. Additionally, the hierarchical stress‐induced martensite twin structure and crystalline‐to‐amorphous phase transformation provide robust toughening mechanisms in the Bio‐HEA. The cytotoxicity test confirms that this Bio‐HEA exhibits excellent biocompatibility without cytotoxicity. In conclusion, this study provides new insights into the development of biomedical alloys with a combination of ultra‐low Young's modulus, excellent biocompatibility, and decent ductility. Simultaneously achieving an ultra‐low Young's modulus, an excellent biocompatibility, and an acceptable ductility possess significant challenges in traditional biomedical alloys. This work presents a generic solution to an ever‐lasting challenge in metal materials design: i.e., achieving low Young's modulus analogous to the human bone while maintaining commendable tensile ductility as well as excellent biocompatibility in a biomedical high‐entropy alloy (Bio‐HEA).