Long noncoding RNAs (lncRNAs) play pivotal roles in tumor development, but little is known about their involvement in the early steps of tumorigenesis. To identify lncRNAs dysregulated in precancerous lesions, we analyzed genome-wide trimethylation of histone H3 lysine 4 (H3K4me3) to screen for transcriptionally active lncRNA genes in the nontumorous gastric mucosa of patients with gastric cancer (GC) and healthy individuals. We found that H3K4me3 at TM4SF1-AS1 is specifically upregulated in GC patients and that expression of TM4SF1-AS1 is prevalently elevated in primary and cultured GC cells. TM4SF1-AS1 contributes to GC cell proliferation in vitro and in vivo, and its oncogenic function is mediated, at least in part, through interaction with Pur-α and YB-1. Chromatin isolation by RNA purification (ChIRP)-mass spectrometry demonstrated that TM4SF1‑AS1 associates with several stress granule (SG)-related proteins, including G3BP2, RACK1 and DDX3. Notably, TM4SF1-AS1 promotes SG formation and inhibits apoptosis in GC cells by sequestering RACK1, an activator of the stress-responsive MAPK pathway, within SGs. TM4SF1‑AS1-induced SG formation and apoptosis inhibition are dependent on Pur-α and YB-1. These findings suggest TM4SF1-AS1 contributes to tumorigenesis by enhancing SG-mediated stress adaptation.