Over-activation of osteoclasts is directly responsible for pathological bone loss in conditions such as rheumatoid arthritis and cancer metastasis to bone. Hypoxia is a common feature of these conditions, associated with poor prognosis, which also stimulates osteoclast-mediated bone resorption via induction of the hypoxia-inducible transcription factor HIF-1 alpha. Here, we investigate the effects of fibroblast growth factor 11 (FGF11) on osteoclast function. FGF11 is an intracellular FGF that was induced both by hypoxia (2% O-2, p < 0.01) and by inhibition of the HIF-regulating prolyl hydroxylase enzymes (CoCl2, p < 0.001) in osteoclasts. Isoform-specific siRNA demonstrated that the induction of Fgf11 mRNA expression by hypoxia is HIF-1 alpha-dependent (p < 0.01). Hypoxic stimulation of bone resorption was inhibited in osteoclasts treated with siRNA targeting FGF11 (p < 0.05). This was at least partially due to reduced secretion of an unidentified pro-resorptive factor downstream of FGF11. FGF11 expression within hypoxic, resorbing osteoclasts co-localised with microtubule-associated alpha-tubulin. FGF11 was also abundantly expressed in osteoclasts within the rheumatoid synovium and in giant cell tumour of bone. This study suggests FGF11 as a novel factor driving pathological bone resorption in osteolytic disease and as a potential target for the development of new anti-resorptive therapeutic agents.