The gonads of sea urchins (Strongylocentrotus intermedius) are characterized by high levels of arachidonic acid (ARA, 20:4n-6) and eicosapentaenoic acid (EPA, 20:5n-3). However, to our knowledge, little information is available regarding the physiological response of adult sea urchins to dietary ARA. In the present study, four dietary feeds were formulated with graded ARA (0, 0.5%, 1%, and 2% dry diet). Each diet was randomly allocated to three cages during a 56-day feeding experiment. The results showed that the sea urchin weight gain rate (WGR) and the gonadosomatic index (GI) significantly increased as ARA was equal to or above 1.0% of dry diet (P < 0.05). The activities of superoxide dismutase (SOD), catalase (CAT), and total anti-oxidative capacity (T-AOC) were the highest in the coelomic fluid of sea urchins that were fed diets with 1% ARA. The total essential amino acid (TEAA) and its ratio to total non-essential amino acid (TNEAA) showed a similar tendency to WGR and GI as dietary ARA increased, and the highest TEAA and TEAA/TNEAA were observed in the gonads of sea urchins that were fed diets with 1% ARA. Levels of ARA and ARA/EPA of the gonads increased while n-3/n-6 polyunsaturated fatty acid (PUFA) decreased with the increase of dietary ARA (P < 0.05). EPA in the gonads of experimental animals fed with formulated feeds showed no significant differences (P > 0.05), but was significantly lower than those fed with kelp (P < 0.05). These results suggested that relatively higher levels of ARA (1% dry diet) significantly promoted growth, gonad development, activities of antioxidant enzymes, as well as nutritional values (TEAA, TEAA/TNEAA, and PUFA) of adult S. intermedius.