Abstract: The market value of sea urchin gonads depends on gonad size and texture. This study investigated the effects of dietary vitamin B₂ on gonad growth, texture, collagen content and related gene expression in Mesocentrotus nudus. Three formulated feeds were supplemented with 0, 15 and 30 mg/kg vitamin B₂; fresh kelp (Saccharina japonica) served as the control. Each diet was randomly assigned to three tanks of M. nudus initial body weight: (3.63 ± 0.02) g. The results indicated that the gonadosomatic index (GSI) and texture traits (hardness, springiness, adhesiveness, gumminess and water content) of M. nudus fed kelp were significantly lower than those fed formulated feeds. The GSI and texture traits of M. nudus fed 15 mg/kg vitamin B₂ were significantly greater than those fed 0 and 30 mg/kg vitamin B₂. The gonad collagen content of sea urchins fed 15 mg/kg vitamin B₂ was significantly higher than those fed kelp, 0 and 30 mg/kg vitamin B₂. The expression levels of collagen synthesis genes (collagen protein alpha-1, 2 and 3) and transforming growth factor beta (TGF-β)/Sma- and Mad-related protein (Smad) pathway genes TGF-β receptor 1 and 2, smad nuclear-interacting protein 1 (SNIP1) and prolyl 4-hydroxylase subunit beta (P4Hβ) of M. nudus fed 15 mg/kg vitamin B₂ were significantly higher than those fed 0 and 30 mg/kg vitamin B₂. On the contrary, the expression levels of collagen degradation genes lysyl oxidase-like 2 (LOXL2) and matrix metalloproteinase 14 (MMP14) in the 15 mg/kg vitamin B₂ group were significantly lower than those in the 0 and 30 mg/kg vitamin B₂ and kelp groups. In conclusion, vitamin B₂ at 15 mg/kg improves gonad texture and collagen content in M. nudus by enhancing collagen synthesis and suppressing degradation through the TGF-β/Smad pathway, providing insights into vitamin B₂ supplementation for gonad quality enhancement.