Aim: To explore the mechanism by which long non-coding RNA (lncRNA) TTN-AS1 regulates osteosarcoma cell apoptosis and drug resistance via the microRNA miR-134-5p/malignant brain tumour domain containing 1 (MBTD1) axis. Results: The lncRNA TTN-AS1 was highly expressed in osteosarcoma and was associated with poor prognosis. The lncRNA TTN-AS1 promoted cell viability and inhibited apoptosis. MiR-134-5p targeted MBTD1, which was regulated by lncRNA TTN-AS1. MBTD1 was highly expressed in osteosarcoma and was associated with poor prognosis. MBTD1 promoted cell viability and inhibited apoptosis, and knockdown of MBTD1 reversed the cancer-promoting effects of lncRNA TTN-AS1. Downregulation of lncRNA TTN-AS1 reduced drug resistance. Conclusion: In osteosarcoma, lncRNA TTN-AS1 promoted the expression of MBTD1 by targeting miR-134-5p and regulated cell growth, apoptosis and drug resistance. Methods: The expression characteristics of genes in osteosarcoma patients were analysed using bioinformatics. Plasmid transfection technology was applied to silence or overexpress lncRNA TTN-AS1, miR-134-5p and MBTD1. Western blotting and quantitative polymerase chain reaction (qPCR) were used to detect protein and RNA, respectively. A cell counting kit 8 (CCK-8) and flow cytometry were used to detect cell viability and apoptosis. The effects of lncRNA TTN-AS1 and MBTD1 on osteosarcoma in vivo were studied by using a tumour burden assay.