Abstract: Objective To investigate the effects of long-chain noncoding RNA AC138128.1 (lncRNA AC138128.1) on the proliferation and apoptosis of gastric cancer cells and the role of microRNA-9 (miR-9). Methods Gastric cancer cells overexpressing lncRNA AC138128.1 and/or miR-9 were constructed through plasmid transfection. The levels of lncRNA AC138128.1 and miR-9 were verified by qRT-PCR. The cellular proliferation ability was detected by CCK8 assay, and the apoptosis was detected by TUNEL assay. The tumorigenicity of gastric cancer cells in vivo was evaluated by nude mouse experiment. Results LncRNA AC138128.1 was lowly expressed in gastric cancer cells. Up-regulation of lncRNA AC138128.1 significantly inhibited the proliferation of gastric cancer cells but promoted their apoptosis. In vivo up-regulation of lncRNA AC138128.1 effectively reduced the tumorigenicity of gastric cancer cells. Up-regulation of miR-9 significantly reversed the effects of lncRNA AC138128.1 on the proliferation and apoptosis of gastric cancer cells. Conclusions LncRNA AC138128.1 may inhibit the proliferation and promote apoptosis of gastric cancer cells by regulating miR-9, suggesting that lncRNA AC138128.1 may play a positive role in the treatment of gastric cancer.