We report a case of synchronous squamous cell carcinoma of the mandibular gingiva and primary malignant lymphoma of the neck. A 51-year-old man was referred to our hospital for further evaluation of an intractable gingival ulcer. Physical examination at initial presentation showed an ulcerative lesion of the left mandibular molar gingiva with an irregular border and induration, which was diagnosed as squamous cell carcinoma on abiopsy. An imaging study showed mandibular invasion of squamous cell carcinoma and metastases to the left cervical lymph nodes. Marginal resection of the mandible and left neck dissection were performed with the patient under general anesthesia. There was no histopathological evidence of cervical lymph node metastasis. However, postoperative physical and radiographic findings at 2 months showed two swollen lymph nodes in the right side of the neck, suggesting early metastases to the contralateral cervical lymph nodes. Right neck dissection was performed with the patient under general anesthesia, and the histopathological diagnosis was malignant lymphoma (marginal zone B-cell lymphoma). The final diagnosis was synchronous squamous cell carcinoma of the mandibular gingiva and primary malignant lymphoma of the neck. The patient was strictly followed up with no additional treatment because of the low malignancy of marginal zone B-cell lymphoma. There has been no evidence of recurrence or metastasis during the 6 years 9 months after the operation.