In the present study, water‐soluble hybrid selenium‐containing nanocomposites have been synthesised via soft oxidation of selenide‐anions, preliminarily generated from elemental bulk‐selenium in the base‐reduction system ‘N ₂ H ₄ –NaOH’. The nanocomposites obtained consist of Se ⁰ NPs (4.6–24.5 nm) stabilised by κ‐carrageenan biocompatible polysaccharide. The structure of these composite nanomaterials has been proven using complementary physical–chemical methods: X‐ray diffraction analysis, transmission electron microscopy, optical spectroscopy, and dynamic light scattering. Optical ranges of ‘emission/excitation’ of aqueous solutions of nanocomposites with Se ⁰ NPs of different sizes are established and the most important parameters of their luminescence are determined. For the obtained nanocomposites, the expressed antiradical activity against free radicals 2,2‐diphenyl‐1‐picrylhydrazyl and 2,2′‐azino‐bis(3‐ethylbenzothiazoline‐6‐sulphonic acid has been found, the value of which depends on the size of selenium nanoparticles. It is experimentally revealed that all obtained nanocomposites are low toxic (LD ₅₀ >2000 mg/kg). It is also found that small selenium nanoparticles (6.8 nm), in contrast to larger nanoparticles (24.5 nm), are accumulated in organisms to significantly increase the level of selenium in the liver, kidneys, and brain (in lesser amounts) of rats.