Background: Low‐level laser irradiation promotes cell viability and wound healing in periodontal tissue. However, its effect on periodontal pathogenic bacteria is unknown. The purpose of this study is to investigate the biologic effect of low‐level laser irradiation on Porphyromonas gingivalis. Methods: A murine macrophage cell line (RAW 264.7) was cultured and treated with gallium‐aluminum‐arsenate (GaAlAs) laser‐irradiated P. gingivalis with varying levels of energy fluency. Gene expression of monocyte chemotactic protein‐1 (MCP‐1), interleukin‐6 (IL‐6), interferon‐β (IFN‐β), and inducible nitric oxide synthase (iNOS) was examined by reverse transcription‐polymerase chain reaction. Production of iNOS was determined by Western blot analysis, and nitric oxide (NO) release was assessed using Griess reagent. Flow cytometric analysis was performed to determine the activation of Toll‐like receptors (TLRs) in response to P. gingivalis. Results: The laser‐irradiated P. gingivalis significantly enhanced messenger RNA and protein levels of iNOS in RAW 264.7. Although the laser irradiation on P. gingivalis did not alter the expression level of MCP‐1, IL‐6, and IFN‐β, it showed a noticeable effect on NO production in RAW 264.7. Furthermore, the laser‐irradiated P. gingivalis accelerated TLR2 activation, but not TLR4 activation. Conclusions: This study reveals that GaAlAs laser irradiation on P. gingivalis induced iNOS expression at the transcriptional and translation levels and increased NO release in macrophages. Moreover, it is confirmed that this process was mediated specifically by TLR2 activation. These findings suggest that low‐level laser irradiation to periodontal pathogenic bacteria could be detrimental to periodontal treatments.