Aims: Investigation of the influence of cultivation time and sea buckthorn press cake (Hippophaë rhamnoides) dosage on mycelium yield of Inonotus obliquus in submerged cultivation and on the yield, monomer composition, and macromolecular properties of the exopolysaccharides (EPS) from culture media and intracellular polysaccharides (IPS) extracted from mycelia. Methods and Results: Supplementation at 5 g l−1 combined with cultivation time of 250 h granted highest yield increase in mycelia (by 122%). The supplementation reduced extraction yield and decreased the molecular weight of the main IPS population. The supplementation increased production and molecular weight of EPS. The relative content of arabinose and rhamnose in EPS positively correlated with dosage of the press cake. The press cake supplementation increased the content of galacturonic acid in IPS, but not in EPS. Conclusion: Sea buckthorn press cake is a food industry fibrous side stream with high oil content. It increases the cultivation yield of Inonotus obliquus mycelium and influences the produced polysaccharides. Significance and Impact of the Study: Mycelium is a resource of bioactive polysaccharides, attracting the interest of nutraceutical companies. Sea buckthorn press cake is a promising supplement for increasing mycelium production. The utilization of this agricultural side stream would therefore favour circular economy.