Purpose: Osteosarcoma (OS), the most common bone malignancy in children, has a poor prognosis, especially when the tumor metastasizes to the lungs. Therefore, novel therapeutic strategies targeting both proliferation and metastasis of OS are required. Podoplanin (PDPN) is expressed by various tumors and is associated with tumor-induced platelet activation via its interaction with C-type lectin-like receptor 2 (CLEC-2) on platelets. We previously found that PDPN contributed to OS growth and metastasis through platelet activation; thus, in this study, we developed an anti-PDPN humanized antibody and evaluated its effect on OS growth and metastasis. Experimental Design: 9 OS cell lines and 2 OS patient-derived cells were collected, and evaluated the efficacy of anti-PDPN-neutralizing antibody, PG4D2 and the humanized anti-PDPN antibody AP201, which had IgG4 framework region. The antitumor and antimetastasis effect of PG4D2 and AP201 were examined in vitro and vivo. In addition, growth signaling by the interaction between PDPN and CLEC-2 was analyzed using phospho-RTK array, growth assay, or immunoblot analysis under the supression of RTKs by siRNA and inhibitor treatment. Results: We observed that PG4D2 treatment significantly suppressed tumor growth and pulmonary metastasis in OS xenograft models highly expressing PDPN. The contribution of PDGFR activation by activated platelet releasates to OS cell proliferation was confirmed, and the humanized antibody, AP201, suppressed in vivo OS growth and metastasis without significant adverse events. Conclusions: Targeting PDPN with a neutralizing antibody against PDPN-CLEC-2 without ADCC and CDC is a novel therapeutic strategy for PDPN-positive OS.