Among basidiomycete molecules, cell wall polysaccharides have been recognized as a major class of bioactive constituents. They are safe molecules and they have a wide spectrum of biological activities, such as immunostimulatory and antioxidant, therefore they possess a prominent role in health benefits coming from mushroom consumption. These properties make mushroom polysaccharides potential candidates for nutraceutical applications and bioactive ingredients production. Fractionation of the hot aqueous extract of Cortinarius caperatus led to isolation of two fractions characterized by spectroscopic analyses (1H-NMR, 13C-NMR, DEPT, 1H-1H COSY, DQCOSY, HSQC, HMBC and HMQC), mass spectrometry (EI-MS, ESI-MS), infrared spectroscopy (FT-IR), chemical reactions of hydrolysis and derivatization followed by GC and HPLC analyses. This mycochemical study revealed a water-soluble fraction characterized as a β-(1-6)-D-glucan, whose presence inside C. capratus has never, to the best of our knowledge, been reported before. Moreover, a water-insoluble fraction purified has been characterized as a branched α-(1-6)-D-glucan with mixed linkages. The antioxidant activity of the soluble polysaccharide fraction has been evaluated as radical-scavenging activity with the DPPH test, the β-(1-6)-D-glucan showed significant antioxidant activity.