BACKGROUND Despite multiple therapeutic modalities, the overall survival of patients with gastric adenocarcinoma remains poor, especially for advanced tumor stages. Although the tyrosine kinase MerTK has shown therapeutic relevance in several tumor entities, its potential effects in gastric adenocarcinoma have not yet been sufficiently characterized. METHODS MerTK expression and its influence on patient survival were evaluated by immunohistochemistry in a cohort of 140 patients with gastric adenocarcinoma. CRISPR/Cas9 knockout and siRNA knockdown of MerTK in the gastric cancer cell lines SNU1, SNU5, and MKN45 was used to analyze protein expression, growth, migration, and invasion properties in vitro and in a murine xenograft model. MerTK was pharmacologically targeted with the small molecule inhibitor UNC2025 in vitro and in vivo. RESULTS In patients, high MerTK expression was associated with decreased overall survival (OS) and lymph node metastasis especially in patients without neoadjuvant therapy (p < 0.05). Knockout and knockdown of MerTK reduced cell proliferation and migration both in vitro and in vivo. UNC2025, a small-molecule inhibitor of MerTK, exhibited a significant therapeutic response in vitro and in vivo. Additionally, MerTK expression attenuated the response to neoadjuvant treatment, and its inhibition sensitized tumor cells to 5-Fluorouracil (5-FU)-based chemotherapy in vitro. CONCLUSIONS Our findings demonstrate the potential value of MerTK as a prognostic biomarker for gastric adenocarcinoma. Targeting MerTK may become a new treatment option, especially for patients with advanced tumors, and may overcome resistance to established chemotherapies.