Six cases of esophageal squamous cell carcinoma were identified in six captive adult Pacific (Phoca vitulina richardsii; n = 2) and Atlantic (Phoca vitulina concolor; n = 4) harbor seals. These seals presented with intermittent dysphagia, regurgitation, inappetence, and abnormal posturing. Common clinical pathology findings in these seals included azotemia, hyperproteinemia, hyperglobulinemia, and leukocytosis. Gastrointestinal endoscopy commonly revealed an ulcerated mass near the gastroesophageal junction. Each seal was euthanized (n = 3) due to poor prognosis, subsequently died while undergoing an anesthetic procedure (n = 2), or found dead (n = 1). The diagnosis of squamous cell carcinoma was confirmed via biopsy of esophageal mucosa during endoscopy or histopathologic examination of affected tissues after necropsy. On the basis of clinical and postmortem findings, esophageal squamous cell carcinoma should be considered as a differential diagnosis in aged harbor seals exhibiting clinical signs of regurgitation, decreased appetite or anorexia, vomiting, and/or abnormal posturing.