The authors report a case of localized nodular tenosynovitis originating from the medial plicae of the knee that was grossly observable, palpable, and symptomatic. A 41-year-old male visited our hospital with a complaint of knee pain that began 4 years ago. He complained of intermittent catching and a mass that appeared in the medial side of the knee 18 months ago. On magnetic resonance imaging, the mass was found on the plicae area of the medial side of the knee, and thus excisional biopsy was performed under arthroscopy. Histological study showed characteristics of localized nodular tenosynovitis originating from the synovium of the knee. At the 1-year postoperative outpatient follow-up, no symptoms or signs of recurrence were found. Since the authors experienced excellent postoperative results of arthroscopically assisted resection of a grossly observable and palpable localized nodular tenosynovitis originating from the synovium of the knee plicae, we report the case with a review of the literature.