Given the limited data in the recent literature regarding spirulina cultivated in Greece, the present study aimed to investigate physicochemical parameters (pH, color), antioxidant activity, and phytochemical content (total phenolic content, quercetin, caffeic acid, C-phycocyanin) of spirulina (Arthrospira platensis) obtained through conventional and organic cultivation methods in different forms (disk-shaped tablets, powder, and flakes) and using different solvents [water and ethanol of grape origin (EEGO)]. Furthermore, the antimicrobial activity of spirulina against selected food pathogens, namely Salmonella Typhimurium, Staphylococcus aureus, Listeria monocytogenes, and Escherichia coli O157:H7, was also investigated. Results showed that the form of spirulina and the solvent used significantly (p < 0.05) affected the physicochemical parameters, antioxidant activity, and phytochemical compounds. Spirulina had a neutral to alkaline pH (6.79 ± 0.02 to 8.24 ± 0.01) and a considerable number of pigments, including C-phycocyanin (0.07 ± 0.01 to 1.65 ± 0.01 mg/mL). The most pronounced effects were recorded in the case of antioxidant activity, where EEGO proved to be the best solvent for the extraction of compounds with antioxidant activity (44.37 ± 2.28% to 48.28 ± 0.39%) against the 2,2-diphenyl-1-picrylhydrazyl (DPPH) radical. In contrast, water proved to be the best solvent for the recovery of total phenolics (4.83 ± 0.55 mg/g DM to 17.07 ± 0.66 mg/g DM), primarily in spirulina powder. Regarding the antibacterial activity, spirulina, primarily in powder and tablet form (in both solvents), showed considerable inhibition zones against the studied food pathogens. The present study presents new knowledge concerning spirulina cultivated in Greece in terms of physicochemical parameters, antioxidant activity, phytochemical content, and antibacterial activity.