Osteosarcoma is an aggressive bone tumor primarily affecting children and adolescents. Its cause is not yet fully understood, and there is an urgent need for more effective treatment. In the present study we identified several miRNAs whose expression is altered in osteosarcoma compared to adjacent normal tissue. Moreover, expression levels of one of those miRNAs, miR-506-3p, correlated negatively with expression of RAB3D (a Ras-related protein). Suppression of miR-506-3p in osteosarcoma led to increased expression of RAB3D, which in turn led to increased CDK4 (cyclin-dependent kinase 4) and MMP9 (matrix metalloprotein 9) activities. Our results suggest that miR-506-3p acts as a tumor suppressor in osteosarcoma and that its downregulation leads to tumor cell proliferation and metastasis due to upregulation of RAB3D- and CDK4-mediated signaling. miR-506-3p thus appears be a potentially useful target for adjuvant therapy in osteosarcoma patients.