In this study, the possibility of preparing new biosorbents by immobilizing the inactive biomass of the fungus Fomitopsis pinicola by incorporating it into calcium alginate was tested. New biosorbents were prepared by liquefying inactive mushroom biomass in 1% and 2% calcium alginate, where the mushroom biomass was added in different amounts in relation to the mass of sodium alginate used in the immobilization process (10%, 20% and 30% of mushroom to the mass of alginate and in ratios of 1:1, 1:1.5 and 1:2 in relation to the mass of sodium alginate). The obtained biosorbents were dried, before and after drying, the diameter and moisture content and degree of swelling of the biosorbent microspheres were determined. Furthermore, the effectiveness of removing the synthetic dye Congo red from aqueous solutions using the obtained biosorbents was tested. The results showed that the addition of inactive mushroom biomass had an effect on increasing the diameter and reducing the water content of the obtained biosorbent microspheres compared to biosorbent microspheres without the addition of mushroom biomass. All biosorbents proved effective for removing Congo red (up to 80%) from water at the selected contact time of 6 h. Biosorbents with a reduced proportion of sodium alginate (1%) and an increased proportion of mushroom biomass also proved to be effective for the removal of Congo red, but the expected increase in efficiency compared to the sample without the addition of mushroom biomass was not achieved. The results obtained from this research point to the possibility of using waste inactive fungus Fomitopsis.pinicola for the preparation of biosorbents, but also to the need for further research that will include more detailed physico-chemical and morphological characterization of biosorbents and more extensive biosorption experiments.