Gastric cancer (GC) is one of the most common devastating and deadly malignancies of the gastrointestinal tract in the world. GLOBOCAN data analysis showed that GC accounted for approximately 1,033,000 new cases of cancer and 78,200 deaths in 2018. Nonstructural maintenance of chromosomes (non-SMC) condensin I complex subunit H (NCAPH) is a regulatory subunit that encodes the non-SMC condensin I complex. Previous studies have demonstrated that NCAPH is highly expressed in multiple cancers. This study aimed to explore the function and potential mechanism of NCAPH in GC. Our study showed that NCAPH expression was significantly upregulated in The Cancer Genome Atlas (TCGA) and Oncomine datasets. Quantitative real-time polymerase chain reaction and western blotting were used to detect NCAPH expression in GC and paracarcinoma tissues. Cell Counting Kit-8 (CCK-8) and colony formation assays were used to examine cell proliferation. Cell scratch and Transwell invasion assays were performed to assess cell migration. In addition, western blotting was used to detect the expression of proteins related to the cell cycle, DNA damage repair, and epithelial-mesenchymal transition (EMT). Flow cytometry was applied for cell cycle and apoptosis detection. A xenograft model was employed to assess the effect of NCAPH in vivo. The results demonstrated that NCAPH expression was significantly increased in GC tissue samples and cell lines. Knockout of NCAPH notably inhibited cell proliferation, cell migration, cell invasion, cell cycle progression, and tumor growth in vitro and in vivo, and induced the G1-phase cell cycle arrest by regulating the DNA damage response. In addition, knockout of NCAPH promoted cell apoptosis and regulated the expression of EMT-related proteins. The results indicate that the knockout of NCAPH in GC cells inhibits proliferation and metastasis via the DNA damage response in vitro and in vivo. NCAPH plays an important role in GC and may be a potential therapeutic target for GC treatment.