This study aimed to prepare silver nanoparticles (AgNPs) using the green microalga Cosmarium sp. by mixing its crude methanolic and hexane extracts with silver nitrate solution (1:1, v/v), as well as assessing their antimicrobial characterization. Alga-mediated AgNPs were characterized by color change. UV-Vis spectroscopy, field emission scanning electron microscopy (FE-SEM), Fourier transform infrared spectroscopy (FTIR), and energy dispersive X-ray diffraction (EDX) analysis were used in this study. This green bio-manufacturing approach led to the formation of AgNPs based on the color-changing evidence after 10-15 min of the mixing process. The UV absorption peaks at 408 nm and 414 nm, for the hexane and methanolic extracts, respectively, confirmed AgNPs formation. FE-SEM and EDX approaches showed that AgNPs particles were rod-shaped and ranged between 50-77 nm in size. For their antibacterial activity, the methanolic nano-extract of Cosmarium sp. was more effective against Klebsiella pneumonia, where the average diameter of the inhibition zone was 20 mm, while the hexane nano-extract exhibited the most antibacterial effect against Salmonella typhi with a diameter range of 22 mm. In addition, the synthesized AgNPs exhibited a prominent antifungal activity, particularly against Mucor racemosus, where the inhibition zones were 23 mm and 30 mm for the algal nano-methanolic and nano-hexane extracts, respectively.