Periodontitis is a common global disease caused by bacterial dysbiosis leading to tissue destruction, and it is strongly associated with anaerobic bacterial colonization. Therapeutic strategies such as oxygen therapy have been developed to positively influence the dysbiotic microbiota, and the use of oxygen-releasing substances may offer an added benefit of avoiding systemic effects commonly associated with antibiotics taken orally or hyperbaric oxygen therapy. Therefore, the oxygen release of calcium peroxide (CaO2) was measured using a dissolved oxygen meter, and CaO2solutions were prepared by dissolving autoclaved CaO2in sterile filtered and deionized water. The effects of CaO2on planktonic bacterial growth and metabolic activity, as well as on biofilms ofStreptococcus oralisandPorphyromonas gingivalis, were investigated through experiments conducted under anaerobic conditions. The objective of this study was to investigate the potential of CaO2as an antimicrobial agent for the treatment of periodontitis. Results showed that CaO2selectively inhibited the growth and viability ofP. gingivalis(p< 0.001) but had little effect onS. oralis(p< 0.01), indicating that CaO2has the potential to selectively affect both planktonic bacteria and mono-species biofilms ofP. gingivalis. The results of this study suggest that CaO2could be a promising antimicrobial agent with selective activity for the treatment of periodontitis.