Background/Objectives: This study explores the bioactive potential of Bovistella utriformis biomass and its polysaccharides (PsBu) through comprehensive biochemical and bioactivity analyses, focusing on their antioxidant, cytotoxic, and antihyperglycemic properties. Methods: Elemental analysis determined the biomass's chemical composition. Antioxidant activity was assessed using ABTS and DPPH assays. Monosaccharide composition was analyzed via gas chromatography-mass spectrometry (GC-MS). In vitro cytotoxicity assays were conducted on cancer and normal cell lines to determine IC50 values and selectivity indices (SI). Zebrafish embryo toxicity was evaluated for teratogenic effects, and an in vivo antihyperglycemic study was performed in diabetic rat models. Results: The biomass exhibited high carbon content (around 41%) and nitrogen levels, with a balanced C/N ratio nearing 5. Protein content exceeded 50%, alongside significant carbohydrate, fiber, and ash levels. Antioxidant assays revealed inhibition rates of approximately 89% (ABTS) and 64% (DPPH). GC-MS analysis identified glucose as the predominant sugar (>80%), followed by galactose and mannose. Additionally, HPLC detected a photoprotective compound, potentially a mycosporin-like amino acid. Cytotoxicity assays demonstrated PsBu's selective activity against colon, lung, and melanoma cancer cell lines (IC50: 100-500 mu g<middle dot>mL(-1)), while effects on normal cell lines were lower (IC50 > 1300 mu g<middle dot>mL(-1) for HaCaT, >2500 mu g<middle dot>mL(-1) for HGF-1), with SI values approaching 27, supporting PsBu's potential as a targeted anticancer agent. Zebrafish embryo assays yielded LC50 values ranging from 1.4 to 1.8 mg<middle dot>mL(-1). In vivo, PsBu reduced fasting blood glucose levels in hyperglycemic rats (approximately 210 mg<middle dot>dL(-1) vs. 230 mg<middle dot>dL(-1) in controls) and preserved pancreatic beta-cell integrity (around 80% vs. 65% in controls). Conclusions: These findings suggest that B. utriformis biomass and PsBu exhibit strong antioxidant activity, selective cytotoxicity against cancer cells, and antihyperglycemic potential, making them promising candidates for further biomedical applications.