The research describes selenium as a widespread element on Earth, present in the lithosphere, atmosphere, hydrosphere, and biosphere, and metabolism of three crucial trace elements (iodine, selenium, and iron), which provide the essential requirements for the functioning and action of the human thyroid gland’s hormonal system. In pregnant women, a deficiency of selenium and iodine increases the risk of chronic diseases in offspring. Selenium exhibits diverse biological activity, such as antitumor effects, cardiovascular system protection, antibacterial or antiviral effects, immunoregulation, and nerve protection, among which the most promising area is antitumor activity. Organic forms of selenium are less toxic than inorganic selenium, such as selenite and selenate. Higher bioavailability implies lower toxicity, which is important for developing new forms of organic selenium compounds. Therefore, the development of less toxic organic selenium compounds is a relevant task in the field of medicine and pharmacology. This research aims to synthesize a selenoorganic compound and investigate the physicochemical properties and biological activity of the newly synthesized selenoorganic compound. The scientific novelty of this work lies in the synthesis and study of the biological activity of the new selenoorganic compound. Thus, to expand the range of selenoorganic compounds, a potassium salt of selenoarsenious acid (KSSVA) has been developed, and research on its physicochemical properties has been conducted. To determine the biological activity of the synthesized selenium preparation, the authors studied toxicity, physical performance, and immunological reactivity in animals.