Gastric adenocarcinomas (GA) is the fourth most common cause of cancer death globally and is a molecularly and phenotypically highly heterogenous disease. (1) Gastrectomy remains the mainstay of curative treatment with perioperative chemoradiotherapy, and in some instances, targeted immunotherapies as adjuncts to the course of treatment. Spontaneous regression (SR), however, defined as partial or complete disappearance of a tumour in the absence of any therapy, remains one of the rarest phenomena in oncology. Estimates suggest an incidence of approximately one in 60,000–100,000 cancer cases, with only sporadic cases reported in the modern literature. Most cases describe SR occurring in renal cell carcinoma, melanoma, and hepatocellular carcinoma, with gastrointestinal sites especially that of gastric being exceedingly rare. However, the pathophysiological mechanisms underlying SR are not fully understood. The most consistently proposed explanation is activation of host immunity. Other hypotheses include ischaemic injury, tumour differentiation, hormonal changes, and alterations in the tumour microenvironment. (2) Here we present a case of SR in a patient with locally advanced, mismatch repair–deficient (dMMR) gastric adenocarcinoma, verified by imaging, endoscopic examination, and formal histopathological review, and discuss possible immunological mechanisms in the context of existing literature.