Objective: This study aimed to investigate the role of fibroblast growth factor-5 (FGF5) in osteosarcoma (OS) and explore the potential mechanisms. Methods: OS gene expression data was downloaded from the Gene Expression Omnibus (GEO; GSE12865) and analyzed by R software. OS tissues and cell lines were collected. The expression level of FGF5 in tumor tissues and cell lines was detected using qRT-PCR. Knockout of FGF5 was performed using CRISPR/Cas9 system. The effects of FGF5 knockout on OS cell proliferation and tumor growth were determined through cell counting kit-8 assay and xenograft nude mice, respectively. Additionally, recombinant FGF5 (rFGF5) was added into OS cell and the effects of rFGF5 on the proliferation and apoptosis of OS cell lines were assayed. Furthermore, the protein expression levels of mitogen-activated protein kinase (MAPK) signaling pathway were detected through Western blot. Results: FGF5 was significantly upregulated in OS tissues and cells, and closely associated with poor differentiation, larger tumor size, lymph node metastasis, and advanced TNM stage. FGF5 knockout could inhibit proliferation of OS cells and tumor growth in nude mouse model. Addition of exogenous rFGF5 promoted OS cell proliferation while inhibited OS cell apoptosis. The expression levels of MAPK signaling pathway proteins in FGF5 knockout group were significantly lower than that in control when there was no rFGF5. Additionally, their expression level in rFGF5 addition group was higher than that in without rFGF5 group. Conclusion: We demonstrated for the first time that FGF5 was overexpressed in OS cell lines and clinical tissue samples and promotes OS cell proliferation by activating MAPK signaling pathway, which indicated that FGF5 was a potential therapeutic target for OS.