In this report, we present the medical history of a 30-year-old male patient with HER2- and PD-L1-negative metastasized adenocarcinoma of the gastric cardia, who received three cycles of pembrolizumab (200 mg every 2 weeks) after the failure of the first-line (1L) treatment with docetaxel, cisplatin, 5‑fluorouracil (DCF). A restaging computed tomography (CT) scan for the chest and abdomen revealed an apparent progressive disease; therefore, the treatment was terminated. Five months after the termination of the treatment, a new CT scan demonstrated a spontaneous treatment response although no treatment was given during this time period, indicating pseudo-progression of the tumor in the first restaging after three cycles of pembrolizumab. This finding is apparently due to the long-term sustainable immunological effects of pembrolizumab. The current report will present this rare case in more detail and summarize the closed and ongoing clinical trials of immunotherapy drugs in gastroesophageal cancer.