BACKGROUND Primary malignant melanoma of the esophagus is a rare disease. However, its exact etiology and progression from melanosis to malignant melanoma have not been elucidated due to its rarity. CASE REPORT We report a case of esophageal melanosis that progressed to malignant melanoma and was synchronous with esophageal squamous cell carcinoma. A male patient in his 60s was diagnosed with right hypopharyngeal cancer. Cervical dissection and chemoradiation therapy were performed. Esophageal melanosis was discovered using gastrointestinal endoscopy during a pre-treatment screening 2 years later and revealed a 0-Ia tumor in the middle thoracic esophagus, coinciding with the esophageal melanosis site. A biopsy revealed malignant melanoma. We performed thoracoscopic total thoracic esophagectomy. The resected specimen showed a 0-Ia lesion, and the invasion depth of the esophageal malignant melanoma was submucosal (pT1b-SM3), N0, Stage I. A 0-IIc lesion was found in the resected specimen [squamous cell carcinoma in situ, intraepithelial mucosal (pTis/T1a-EP), N0, Stage 0]. The patient has been recurrence-free for 18 months post-surgery without postoperative adjuvant chemotherapy and is still receiving outpatient followup. CONCLUSIONS The close relationship between esophageal melanosis and primary malignant melanoma of the esophagus has implicated the melanosis as the origin of the malignant melanoma. The coexistence of esophageal melanosis and esophageal cancer warrants improved patient followup, including biopsy and multiple endoscopic examinations after esophageal melanosis diagnosis.