This study aimed to investigate the role of long non-coding RNA maternally expressed gene 3 (lncRNA MEG3) in chemosensitivity of osteosarcoma (OS), and to reveal the possible underlying mechanisms. In this study, we found that the expression of lncRNA MEG3 was significantly lower in OS tissues and cell lines. Furthermore, lncRNA MEG3 overexpression enhanced chemosensitivity of OS by inhibiting cell proliferation, migration, autophagy, and promoting antitumor immunity. LncRNA MEG3 functioned as miR-21-5 sponge to regulate p53 expression in OS. Mechanically, lncRNA MEG3 promoted OS chemosensitivity by regulating antitumor immunity via miR-21-5p/p53 pathway and autophagy. Collectively, this study provided the evidence that lncRNA MEG3 might be a promising therapeutic target for OS chemoresistance.