Osteopontin (OPN) is a matricellular protein highly expressed in cancer cells, which is able to modulate tumorigenesis and metastasis in several malignancies, including follicular cell-derived thyroid cancers (TC). OPN is one of the gene products aberrantly expressed in TC, but the contribution of each OPN isoform (OPNi), named as OPNa, OPNb and OPNc, is currently unknown. This study aims to analyze the expression profile of OPNi in TC tissue samples, correlate its expression with molecular and clinicopathologic features, and evaluate the role of OPNi in TC cell lines. The expression profiles of OPNi in TC cell lines and in thyroid tissue samples were evaluated by q-RT-PCR and immunohistochemistry. In order to address the putative roles of OPNi in TC, we overexpressed OPNi in TC cell lines. c643 and 8505c cells were transfected with vectors containing OPNi as well as empty vector, and stable overexpressing clones were selected with geneticin. Functional assays (proliferation, migration, motility, gelatin zymography and invasion) were also performed. We found that the OPNa and the OPNb isoforms are expressed in higher levels in classic papillary thyroid carcinoma (cPTC) samples than in non-tumoral thyroid, adenomas and follicular thyroid carcinoma tissues. Conversely, OPNc isoform transcript levels are similar among samples from the aforementioned pathologies. In cPTC samples, high OPNa and OPNb expression levels were significantly associated with higher tumor size, presence of vascular invasion and BRAFV600E mutation. In eight distinct TC cell lines, we observed differential expression of the three OPNi, of which cell line c643 expressed the lower levels of OPNi. Higher proliferation, migration and motility were associated with c643 and 8505c cells overexpressing OPNa. In both cell lines overexpressing OPNa, we observed an increase of matrix metalloproteinase 2 in the extracellular medium. Further, in in vivo CAM assay we found that cells overexpressing OPNa are significantly more invasive when compared to the control cells. Taken together, our data indicate that both OPNa and OPNb are overexpressed in cPTC samples and OPNa is the isoform that is significantly associated to promotion of cell growth, migration, motility and invasion in TC cells.