The article examines the possibility of environmentally safe bioconversion of buckwheat straw as a secondary raw material of the agro-industrial complex. The aim of the study was to evaluate the efficiency of extraction, thermal, and enzymatic processing methods for converting straw into biologically active substances, dietary fibers, and biotechnological products. It was established that about 6.4 t of straw is formed per 1 ha of crops, from which up to 14% of biologically active compounds can be extracted, while up to 86% of the mass is preserved and remains suitable for further processing. Thermal treatment at 100–122 °C promotes the breakdown of the lignocellulosic complex and increases the fiber content to 68.5%. Enzymatic processing ensures a polysaccharide hydrolysis degree of up to 50–55% and utilization of protein components at a level of 43–68%. It is shown that modified enzyme systems increase cellulolytic activity by 38–60%, whereas nitrosamines and pesticides reduce the proteolytic activity of enzymes by 36–50%. The obtained results confirm the prospects of using buckwheat straw for the production of functional ingredients and biotechnological products. Comparative analysis with published data on wheat, barley, and rice straw shows that buckwheat straw, with a cellulose content (45.0%) comparable to wheat straw (35–45%) and exceeding that of barley (37.5%) and rice straw (30–38%), contains a significantly higher level of bioactive compounds (≈14%, mainly flavonoids such as rutin, which is absent in cereal straw) and a moderate lignin content (21.6%). Therefore, it is a promising raw material not only for dietary fiber and bioethanol production but also for obtaining pharmacologically valuable flavonoid compounds.