It is well known that the differential expression of long noncoding RNA (lncRNA) plays critical roles in carcinogenesis. However, the significance of lncRNA in the diagnosis of gastric cancer (GC) remains largely unknown. The goal of this study was to determine the expression level of AC138128.1, a novel lncRNA, in GC and its clinical association. The expression of AC138128.1 in 94 pairs of tumors and normal adjacent tissues (NAT) from patients with GC was detected by quantitative real-time reverse transcription polymerase chain reaction. The potential association between AC138128.1 expression level in GC tissue and clinicopathological features was analyzed. Finally, a receiver operating characteristic (ROC) curve was constructed. The results showed that the expression level of AC138128.1 in GC was significantly decreased compared to that in NAT (p < 0.001). Levels of AC138128.1 in the GC cell lines, AGS BGC-823, HG27, and SGC-7901 were lower than those in the human normal gastric epithelial cell line GES-1. Moreover, the area under the ROC curve was 0.688. This study demonstrates that lncRNA AC138128.1 might be a novel biomarker for predicting GC.