Kaempferol is a natural flavonoid that possesses anti-proliferative and apoptosis-inducing activities in several cancer cell lines. In the present study, we investigated the anti-metastatic activity of kaempferol and its molecular mechanism(s) of action in human osteosarcoma cells. Kaempferol displayed inhibitory effects on the invasion and adhesion of U-2 osteosarcoma (OS) cells in a concentration-dependent manner by Matrigel Transwell assay and cell adhesion assay. Kaempferol also inhibited the migration of U-2 OS cells in a concentration-dependent manner at different treatment time points by wound-healing assay. Additional experiments showed that kaempferol treatment reduced the enzymatic activities and protein levels of matrix metalloproteinase (MMP)-2, MMP-9 and urokinase plasminogen activator (uPA) by gelatin and casein-plasminogen zymography assays and western blot analyses. Kaempferol also downregulated the mRNA levels of MMP-2 and MMP-9 by quantitative PCR analyses. Furthermore, kaempferol was able to reduce the protein phosphorylation of ERK, p38 and JNK by western blotting. By electrophoretic mobility-shift assay (EMSA), we demonstrated that kaempferol decreased the DNA binding activity of AP-1, an action likely to result in the reduced expression of MMP-2, MMP-9 and uPA. Collectively, our data showed that kaempferol attenuated the MAPK signaling pathways including ERK, JNK and p38 and resulted in the decreased DNA binding ability of AP-1, and hence, the downregulation in the expression and enzymatic activities of MMP-2, MMP-9 and uPA, contributing to the inhibition of metastasis of U-2 OS cells. Our results suggest a potential role of kaempferol in the therapy of tumor metastasis of OS.