Periodontitis is a common chronic inflammatory disease associated with biofilm formation, gingival recession, and supporting bone loss that can lead to the formation of periodontal pockets and, ultimately, tooth loss. Clinical treatment for periodontitis through scaling and antibiotics still faces the problems of unavoidable bleeding, injury to periodontal tissue, drug resistance, and insufficient treatment. Herein we prepared an injectable anti-periodontitis ointment with catalytic activity that consists of Pt nanocluster (PtNC) modified g-C3N4 (CN), and PEG400/PEG4000, which efficiently treated biofilm-infected periodontitis. PtNCs (<2 nm) with ultralow content (0.07%) were formed on the surface of CN using mild ultraviolet (UV) irradiation. Due to the strong O2 adsorption and activation ability of CN-PtNCs and their mutual electron transfer, they show both oxidase-like and peroxidase-like activities and produce reactive oxygen species (ROS) in the dark. CN-PtNCs showed strong biofilm elimination ability towards Staphylococcus aureus (S. aureus) and Escherichia coli (E. coli). Furthermore, benefiting from the good biocompatibility of CN-PtNCs and the injectable property of the PEG400/PEG4000 ointment, the CN-PtNC ointment with high bioavailability successfully treated periodontitis in rats, alleviating inflammation and reducing bone loss, and showed better performance than periocline. Therefore, this catalytic system is promising for an efficient, non-invasive, and antibiotic-free treatment of periodontitis.