It is widely acknowledged that among the most important global oral health issues are tooth decay and gum infections. Tannins from the stem bark of Phyllanthus columnaris (PCTs) are proposed as a potential antimicrobial treatment against various oral pathogens. This study aims to investigate the antimicrobial activity of PCTs against selected oral pathogens, including five bacterial species (Streptococcus salivarius ATCC 13419, Streptococcus oralis ATCC 6249, Streptococcus mutans ATCC 25175, Porphyromonas gingivalis ATCC 33277, and Fusobacterium nucleatum ATCC 25586) and five Candida species (C. albicans ATCC 14053, C. parapsilosis ATCC 22019, C. tropicalis ATCC 750, C. krusei ATCC 6258, and C. glabrata ATCC 2001). The antimicrobial activities of PCTs were assessed using three techniques: disc diffusion assay (DDA), determination of the minimum inhibitory concentration and minimum bactericidal/fungicidal concentration (MIC/MBC/MFC). Anti-biofilm and antiadherence activities were evaluated using crystal violet and glass surface assays. The time-kill assay was conducted to examine the kinetic effects of PCTs on bacterial growth. Scanning electron microscopy (SEM) and transmission electron microscopy (TEM) were employed to observe changes in the external morphology and internal structure of bacteria after exposure to PCTs. The chemical composition of PCTs was analysed by direct infusion mass spectrometry. Real-time polymerase chain reaction (RT-qPCR) was used to confirm the expression of affected outer membrane genes (RgpA, RgpB, and Kgp) in P. gingivalis. Results from the DDA indicated that PCTs exhibited strong antibacterial activity against S. mutans, S. salivarius, and S. oralis, with inhibition zones of 13.00±0.2, 14.00±0.1, and 12.00±0.1 mm, respectively. For Candida spp., notable inhibition zones were observed for C. glabrata, C. tropicalis, C. albicans, C. parapsilosis, and C. krusei, with zones measuring 20.00±0.7, 17.00±0.7, 10.00±0.5, 11.00±0.5, and 9.00±0.7 mm, respectively. PCTs inhibited the growth of all tested pathogens with MIC values ranging from 0.16 to 1.25 mg/mL. Gram-negative anaerobes had the lowest MBC values at 0.32 mg/mL, compared to Gram-positive cariogenic bacteria (2.5->5.0 mg/mL). Concentrations producing a 50% reduction in anti-biofilm and anti-adherence activities were 3.12 mg/mL and 6.25 mg/mL, respectively, for all bacteria except S. oralis. The time-kill kinetic analysis revealed that PCTs demonstrated concentration-dependent bactericidal activity. SEM and TEM analyses showed that treated cells of P. gingivalis and F. nucleatum experienced structural damage, primarily targeting the bacterial plasma membrane. RT-qPCR confirmed that PCTs affected the genes RgpA, RgpB, and Kgp in the outer membrane of P. gingivalis. In conclusion, this study highlights the potential of PCTs as a promising therapeutic agent against oral pathogens.