In the present investigation, Spirulina platensis extract was used for biosynthesis of silver nanoparticles (AgNPs) and the antibacterial activity of the biosynthesized nanoparticles against oral pathogens was assessed. The biosynthesis of AgNPs was successfully achieved using 0.5 ml of Spirulina extract and 50 ml of silver nitrate (1mM) after 5 minutes of heating at 50°C. The formation of AgNPs was confirmed using UV- Visible spectropho-tometer by the appearance of typical plasmon resonance band of AgNPs at 425nm. The average particle size of the biosynthesized AgNPs was 25.56 nm. Three Gram positive oral bacteria were used to investigate the antibacterial activity of biosynthesized silver nanoparticles. These bacteria were Streptococcus mutans, Staphylococcus aureus and Enterococcus faecalis. The antibacterial activity was studied by determining the inhibition zone diameter using the disc diffusion method. The results showed that the AgNPs biosynthesized using Spirulina extract have good antibacterial activity against the three tested bacteria. The inhibition zone diameters were 12 mm, 12 mm and 13 mm for Streptococcus mutans, Staphylococcus aureus and Enterococcus faecalis, respectively. In conclusion, AgNPs biosynthesized using Spirulina platensis extract could be employed as antibacterial materials in dentistry. Further work could be addressed towards the optimization of the preparation conditions and controlling the size of nanoparticles.