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Fish reproduction is closely related to the regulation of the brain and liver, making it essential to identify the factors that control this process. The turbot (<i>Scophthalmus maximus</i>) is an economically significant species that has been successfully breeding through industrial aquaculture. Investigation of factors into the involvement of gonadal development is crucial for artificial breeding. In this study, a new insulin-like growth factor 3 <i>igf3</i> gene was cloned and characterized. Additionally, all three types of turbot IGFs contain a distinct IGF domain, with IGF3 and IGF2 being grouped with other teleosts, demonstrating a closely related genetic relationship. The expression analysis showed that <i>igf3</i> mRNA is predominantly expressed in the gonad and brain (specifically in the pituitary and hypothalamus), suggesting its effects at multiple levels in the brain–pituitary–gonadal axis. Furthermore, the mRNA levels of <i>igfs</i> during gonadal development were examined. In the gonad and liver of female turbots, the expression levels of <i>igfs</i> mRNA significantly increased from stage II to VI during the process of oogenesis, including maturation and degeneration. In the testis and liver of male turbots, <i>igf3</i> mRNA maintained high expression levels during the proliferation of spermatogonia at stages II and III. In addition, the highest levels of <i>igf1</i> and <i>igf2</i> were observed at the beginning of spermatogenesis and during sperm production at stages V and VI. All the results suggest that the IGFs were closely related to the gonadal development in turbot and improve a better understanding of the IGF system in the regulation of gonadal development in teleost.