Maintaining sustainable production of healthy and nutritious food represents a major global challenge. Cultivated meat could reduce the environmental impact of livestock production, but it remains costly, largely due to the expense of fetal bovine serum, a common cell culture media supplement. Algal and cyanobacterial proteins, particularly phycobiliproteins, represent promising sustainable sources of potentially bioactive peptides that may contribute to the development of serum-reduced or serum-free culture media. This study investigated the potential of hydrolysates prepared using the protein extracts of the red alga Porphyra haitanensis and the cyanobacterium Arthrospira platensis as functional supplements for cell cultivation in a low-serum environment. Twelve hydrolysates were generated by treating protein extracts from Arthrospira platensis and Porphyra haitanensis with six proteolytic enzymes: pepsin, trypsin, chymotrypsin, subtilisin, thermolysin and proteinase K. The effects of these hydrolysates were studied in three cell models of mammalian (CHO-K1), avian (QM7), and fish (MACK1) origin. The effects of hydrolysates on cell viability, proliferation, and antioxidant activity were evaluated. A. platensis–chymotrypsin hydrolysate significantly improved QM7 cell viability compared to control in both 80% reduced-serum and tryptose phosphate broth conditions (p ≤ 0.0001). Additionally, peptide composition analysis and identification of amino acid sequences provided preliminary insights into potentially bioactive chromopeptides associated with the maintenance and promotion of cellular growth. Algae/cyanobacteria-derived hydrolysates positively regulated cell viability and proliferation in serum-reduced conditions, although effects were dependent on the algal species, enzymatic treatment, and cell type. These findings highlight their potential as sustainable supplements for alternative cell culture media.