Background: Gastric cancer (GC) is the third leading cause of cancer death. Early detection is a key factor to reduce its mortality. Methods: We retrospectively collected pre-and postoperative serum samples as well as tumour tissues and adjacent normal tissues from 100 GC patients. Serum samples from non-cancerous patients were served as controls (n = 50). A high-throughput protein detection technology, multiplex proximity extension assays (PEA), was applied to measure levels of over 300 proteins. Alteration of each protein was analysed by univariate analysis. Elastic-net logistic regression was performed to select serum proteins into the diagnostic model. Findings: We identified 19 serum proteins (CEACAM5, CA9, MSLN, CCL20, SCF, TGF-alpha, MMP-1, MMP-10, IGF-1, CDCP1, PPIA, DDAH-1, HMOX-1, FLI1, IL-7, ZBTB-17, APBB1IP, KAZALD-1, and ADAMTS-15) that together distinguish GC cases from controls with a diagnostic sensitivity of 93%, specificity of 100%, and area under receiver operating characteristic curve (AUC) of 0·99 (95% CI: 0·98-1). Moreover, the 19-serum protein signature provided an increased diagnostic capacity in patients at TNM I-II stage (sensitivity 89%, specificity 100%, AUC 0·99) and in patients with high microsatellite instability (MSI) (91%, 98%, and 0·99) compared to individual proteins. These promising results will inspire a large-scale independent cohort study to be pursued for validating the proposed protein signature. Interpretation: Based on targeted proteomics and elastic-net logistic regression, we identified a 19-serum protein signature which could contribute to clinical GC diagnosis, especially for patients at early stage and those with high MSI. Fund: This study was supported by a European H2020-Marie Skłodowska-Curie Innovative Training Networks grant (316,929, GastricGlycoExplorer). Funder had no influence on trial design, data evaluation, and interpretation.EBioMedicine 44 (2019) 322-333Abbreviations: ADAMTS-15, A disintegrin and metalloproteinase with thrombospondin motifs 15; APBB1IP, Amyloid beta A4 precursor protein-binding family B member 1-interacting protein; CA9, Carbonic anhydrase 9; CCL20, C-C motif chemokine 20; CDCP1, CUB domain-containing protein 1; CEACAM5, Carcinoembryonic antigen-related cell adhesion molecule 5; DDAH1, dimethylarginine dimethylaminohydrolase 1; FLI-1, Friend leukemia integration 1 transcription factor; GCNT1, beta-1,3-galactosyl-O-glycosyl-glycoprotein beta-1,6-Nacetylglucosaminyltransferase; HMOX1, Heme oxygenase 1; IGF1, Insulin-like growth factor I; IL-7, Interleukin-7; KAZALD1, Kazal-type serine protease inhibitor domain containing protein 1; MSLN, Mesothelin; MMP-1, Matrix metalloproteinase-1; MMP-10, Matrix metalloproteinase-10; PPIA, Peptidyl-prolyl cis-trans isomerase A; SCF, Stem cell factor/KIT ligand; TGF alpha, Transforming growth factor alpha; ZBTB-17, Zinc finger and BTB domain-containing protein 17.