Gastrointestinal (GI)-neuroendocrine neoplasms (NENs) are subclassified in neuroendocrine tumors (NETs), neuroendocrine carcinomas (NECs), and mixed neuroendocrine-non-neuroendocrine neoplasms (MiNENs). The genetic characteristics of GI-NEN has been a hot issue in recent years, but more studies are needed to provide further details. This study aims to provide additional data about genomic characteristics of GI-NENs and the genetic differences between NETs and NECs.Thirteen samples were selected for next-generation sequencing (NGS) analysis with a 425-gene panel. Microsatellite instability (MSI) and tumor mutational burden (TMB) were calculated as well as immunohistochemistry (IHC) was used to test for protein expression.Genetic alterations were very common in NECs, but rare in NETs. The average TMB of NETs and NECs was 2.3 and 6.9, respectively. The TMB of NECs was significantly higher compared to NETs. The TP53 mutation rate was significantly higher in NECs than in NETs (100% vs. 20%), other mutations involved MTOR (n = 2, 15.4%), DDR2 (n = 3, 23.1%), ERBB4 (n = 1, 7.7%), BRCA1 (n = 1, 7.7%), BRCA2 (n = 1, 7.7%), ATM (n = 1, 7.7%), and SMAD4 (n = 1, 7.7%). Deep loss of SMAD4 (1/3, 33.3%), SDHB (1/3, 33.3%), RB1 (1/3, 33.3%), and BRCA2 (1/3, 33.3%), high-level amplification of CRKL (1/3, 33.3%), CCNE1(1/3, 33.3%), and MCL1(1/3, 33.3%) were found in NECs. The integrated analysis found these genetic alterations frequently involve DNA repair and cell cycle, PI3K/AKT/mTOR and TGF-β/SMAD4 signaling pathways.Genetic alterations were very common in NECs and rare in NETs, and frequently involved three main signaling pathways. NEC patients harboring these genetic alterations may benefit from targeted therapy and PD-1/PD-L1 immunotherapy.© 2023 The Authors. Cancer Medicine published by John Wiley & Sons Ltd.