The aim of this study was to investigate whether long intergenic non‐coding RNA 1929 (LINC01929), a novel long non‐coding RNA, could serve as a prognostic biomarker for various tumours and explore its function. The expression and prognosis of LINC01929 across 33 different tumour types in patients in the Cancer Genome Atlas (TCGA) database were analysed. Also, the correlation between LINC01929 expression, tumour mutational burden (TMB), microsatellite instability (MSI), immune checkpoint status and immune cell infiltration was examined. Moreover, the function of LINC01929 in the breast cancer cell lines was explored via CCK‐8, colony formation and cell cycle assays. In addition, the downstream mechanisms of LINC01929 were analysed via transcriptome sequencing, RT–qPCR, and western blotting. Our analysis revealed that LINC01929 was weakly expressed in 3 tumour types and highly expressed in 14 tumour types, and low expression of LINC01929 was correlated with better clinical outcomes in 15 tumour types. Furthermore, LINC01929 expression was correlated significantly with the TMB, MSI, immune checkpoint and immune cell infiltration across multiple tumour types. The knockdown of LINC01929 inhibited cell cycle progression, cell proliferation, and tumorigenesis and downregulated the TNF pathway and STAT3 expression. The treatment with exogenous TNF‐α partially reversed the cell cycle progression and proliferation inhibition caused by LINC01929 knockdown, and these effects were accompanied by changes in STAT3 expression. LINC01929 may serve as an effective biomarker affecting the TMB, MSI, immune cell infiltration and immune checkpoint status. Mechanistically, LINC01929 affects cell cycle progression and cell proliferation through the TNF/STAT3 axis. These findings offer valuable insights into the potential applications of LINC01929 in tumour therapy, which may yield novel targets and strategies for the diagnosis and treatment of patients.