Japanese red pine (Pinus densiflora Sieb. et Zucc.) was reported to have strong allelopathic activity. However, the allelopathic activity of the pine has not been monitored throughout a year. In this research, the seasonal change in the allelopathic activity of the extracts of Japanese red pine needles was determined. The extracts of pine needles inhibited the growth of cress (Lepidium sativum L.) and the inhibitory activity of the extracts showed the seasonal variation. The inhibitory activity of June extract was significantly greater than that of December extract. The extracts also inhibited the barnyard grass (Echinochloa crus-galli (L.) Beauv.), but the seasonal change was not apparent. The present research suggests Japanese red pine needles have allelopathic activity throughout a year and the patterns of the change in the inhibitory activity were different with cress and barnyard grass, which may be due to the different sensitivity of cress and barnyard grass to a mixture of allelopathic substances in the pine needles.