β-catenin is a key signalling molecule in the canonical Wnt pathway, which plays a role in cell adhesion, embryogenesis and sex determination. However, little is known about its function in teleosts. We cloned and characterized the full-length β-catenin1 gene from half-smooth tongue sole (Cynoglossus semilaevis), which was designated CS-β-catenin1. The CS-β-catenin1 cDNA consists of 2,346 nucleotides and encodes a protein with 782 amino acids. Although CS-β-catenin1 was transcribed in the gonads of both sexes, the level was significantly higher in ovaries compared to testes. Furthermore, the mRNA level of CS-β-catenin1 was significantly upregulated at 160 days and constantly increased until 2 years of age. In situ hybridization revealed that CS-β-catenin1 mRNA was mainly localized in oocyte cells, especially in stage I, II and III oocytes. When CS-β-catenin1 expression was inhibited by injection of quercetin in the ovaries, levels of CS-Figla and CS-foxl2 mRNA were significantly down-regulated, and CS-dmrt1 was up-regulated, which suggested that CS-β-catenin1 is a potential upstream gene of CS-Figla and is involved in the development of the ovaries, i.e., folliculogenesis.