Restoring degraded soils and improving soil quality in agroecosystems are key objectives of the United Nations 2030 Agenda for Sustainable Development. To improve soil quality, modern agriculture mostly relies on inorganic soil fertilizers, which undoubtedly improve crop yields and some soil characteristics, but adversely affect the environment. Consequently, there is a growing demand for eco-friendly alternatives. Biopolymers derived from bacteria and biomass of cyanobacteria and marine algae could be suitable soil improvers. However, bibliographic research using the entire Scopus database revealed that only a few but promising studies focused on biopolymers/biomasses as improvers of degraded soils. The present study aimed to evaluate the effects of biopolymer/biomass addition on soil quality, assessed through several microbial and chemical properties. As part of the PRIN PNRR 2022M7S2J SeaForSoil Project, a pot experiment was conducted by adding a biopolymer or biomass to the soil (10 g/kg). The experimental design included 5 treatments: exopolysaccharide (EPS) from the thermophilic Geobacillus thermodenitrificans B3-72; biosurfactant (BS) from the thermophilic Bacillus sp. s7s-3ng; biomasses from the seaweed Ulva ohnoi (BM); the cyanobacterium Leptolyngbya sp. (BC); a control soil, without any additive. After 60 days of exposure to the biopolymer/biomass in a thermostatic chamber (25 ± 3°C), the following properties were determined: fungal biomass, potential soil respiration, nitrogen mineralization, nitrification, water holding capacity, pH, cation exchange capacity, and ammonium and nitrate contents. Without causing any negative effects on the soil, the addition of each biopolymer/biomass improved most of the considered parameters, with the best results observed in soil treated with EPS or BM. The data suggest that the biopolymer/biomass could be suitable for the restoration of degraded soils. However, further studies are needed to further investigate the effect of the biopolymer/biomass in different soil types and in-field conditions.