Growth-arrest-specific transcript 5 (GAS5) functions as a tumor suppressor in a variety of cancers. GAS5 has been reported to be down-regulated in oral squamous cell carcinoma (OSCC). The aim of this study was to investigate the mechanisms of how GAS5 acts as a tumor suppressor in OSCC. qRT-PCR, cell viability, wound-healing, and transwell assays showed that knockdown of GAS5 increased miR-21 expression and promoted proliferation, migration, invasion, and epithelial-mesenchymal transition of OSCC cells. In contrast, overexpression of GAS5 showed the opposite effects. Furthermore, miR-21 overexpression reversed the effect of GAS5. Western blot showed that knockdown of GAS5 suppressed PTEN, while phosphorylation of Akt was promoted. PCNA, cyclinD1, and Ki-67 were up-regulated, indicating enhanced proliferation. E-cadherin was down-regulated, while N-cadherin, vimentin, and snail1 were increased, indicating augmented epithelial-mesenchymal transition. Overexpression of GAS5 regulated these proteins inversely. Overexpression of miR-21 reversed the effect of GAS5 on these proteins. Taken together, GAS5 suppresses proliferation, migration, invasion, and epithelial-mesenchymal transition in OSCC through the miR-21/PTEN axis and might be a novel therapeutic target for OSCC.