Background: Tumor metastasis is an important factor in treatment failure for advanced gastric cancer. Family with sequence similarity 3 member C (FAM3C) is known to play a critical role in inducing epithelial–mesenchymal transition in several cancer types, while its role in gastric cancer is unidentified. The aim of this study was to investigate the role of FAM3C in gastric cancer and provide new information on the receptor tyrosine-kinase pathway and cytokine-based therapies. Methods: FAM3C expression was tested in human gastric cancer tissue and adjacent normal mucosa, and the prognostic effect of FAM3C was analyzed in data from the Cancer Genome Atlas (TCGA). The role of FAM3C in gastric cancer proliferation and metastasis was investigated in vitro and in vivo. Western blot analysis and immunofluorescence were used to detect the underlying mechanisms. Results: FAM3C expression was increased in gastric cancer tissue and showed cytoplasmic distribution. Gastric cancer patients with FAM3C overexpression had significantly worse prognoses based on TCGA data. In the gastric cancer cell lines MKN45 and AGS, knockdown of FAM3C dramatically attenuated cell migration, but had almost no influence on proliferation, while exogenous FAM3C promoted cell migration in a cell line with low FAM3C expression. Kyoto Encyclopedia of Genes and Genomes (KEGG) pathway enrichment of TCGA data showed that FAM3C was mainly associated with genes involved in focal adhesion, extracellular matrix–receptor interactions and the PI3K–Akt signaling pathway. Knockdown of FAM3C in gastric cancer cell lines significantly suppressed epithelial–mesenchymal transition, as demonstrated by increased expression of E-cadherin and decreased expression of Snail and Slug. Furthermore, knockdown of FAM3C strongly suppressed activation of the PI3K–Akt signaling pathway. Finally, we confirmed that FAM3C knockdown significantly decreased metastatic lesions in vivo. Conclusion: Our study demonstrated that FAM3C can promote gastric cancer metastasis both in vitro and in vivo. FAM3C should be taken into consideration for gastric cancer treatments involving inhibition of the ligands and downstream pathways of receptor tyrosine kinases. © 2018 Shi et al.