Osteosarcoma (OS) is the most representative primary bone tumour in children and teenagers. This study explored the regulatory effects of long noncoding RNA MIR503HG (MIR503HG) on the biological functions of OS cells, and further investigated the potential mechanism of MIR503HG function exertion by analyzing the microRNA-103a-3p (miR-103a-3p) in OS cells and tissues. The expression of MIR503HG was examined using reverse transcription-quantitative PCR. OS cell proliferation was assessed by CCK-8 assay. Transwell assay was used to evaluate the migration and invasion of OS cells. The interaction between MIR503HG and miR-103a-3p was detected using the Dual-luciferase reporter assay. Forty-six paired OS tissues were collected, and the expression and correlation of MIR503HG and miR-103a-3p were evaluated. The expression of MIR503HG were significantly decreased in both OS cells and tissues. Over-expression of MIR503HG inhibited OS cell proliferation, migration and invasion. miR-103a-3p was directly targeted by MIR503HG in OS cells, and mediated the inhibitory effects of MIR503HG on OS cell malignant behaviors. miR-103a-3p expression was upregulated in OS tissues, which was negatively correlated with MIR503HG expression levels. The expression of MIR503HG was associated with OS patients' tumor size, differentiation, distant metastasis and clinical stage. Decreased MIR503HG in OS tissues and cell lines served as a tumor suppressor by inhibiting OS cell malignant behaviors through sponging miR-103a-3p. The findings of this study may provide evidence for the development of novel therapeutic targets of OS.