Lens growth involves the proliferation of epithelial cells, followed by their migration to the equator region and differentiation into secondary fiber cells. It is widely accepted that fibroblast growth factor (FGF) signaling is required for the differentiation of lens epithelial cells into crystallin-rich fibers, but this signaling is insufficient to induce full differentiation. To better understand lens development, investigatory and functional analyses of novel molecules are required. Here, we demonstrate that Equarin, which is a novel secreted molecule, was expressed exclusively in the lens equator region during chick lens development. Equarin upregulated the expression of fiber markers, as demonstrated using in ovo electroporation. In a primary lens cell culture, Equarin promoted the biochemical and morphological changes associated with the differentiation of lens epithelial cells to fibers. A loss-of-function analysis was performed using zinc-finger nucleases targeting the Equarin gene. Lens cell differentiation was markedly inhibited when endogenous Equarin was blocked, indicating that Equarin was essential for normal chick lens differentiation. Furthermore, biochemical analysis showed that Equarin directly bound to FGFs and heparan sulfate proteoglycan and thereby upregulated the expression of phospho-ERK1/2 (ERK-P) proteins, the downstream of the FGF signaling pathway, in vivo and in vitro. Conversely, the absence of endogenous Equarin clearly diminished FGF-induced fiber differentiation. Taken together, our results suggest that Equarin is involved as an FGF modulator in chick lens differentiation.