Melanotic nerve sheath tumor (MNST) is a rare variant of schwannoma. Here, we report an unusual case of multiple MNST lesions located in the upper limb nerves. The patient presented with a mass on the left wrist in 2016 and another mass on the left thumb in 2017. In both instances, magnetic resonance imaging scans confirmed multiple giant-cell tumors of the tendon sheath. Persistent pain in the left upper limb and numbness in the ring finger and little finger recurred in 2021. High-frequency ultrasound (HFUS) showed that the left brachial plexus nerves (C5-8) were widened compared with those on the contralateral side; the neuroma formed at the lateral cord, and the median nerve was markedly thickened. The surgical findings were consistent with the ultrasound results. Pathology confirmed that the tumors were malignant MNSTs. HFUS is important for preoperative diagnosis and lesion localization, even identifying some lesions that are unrecognized on magnetic resonance imaging; thus, HFUS is crucial for improving surgical strategy and decision-making.