An 8-week feeding experiment was conducted to evaluate the effects of galactomannan oligosaccharides (GMOS) on the growth, digestion, nutritional composition of the body wall and immunity of ). A basal diet of 38% crude protein and approximately 1.6% crude lipid was used as the control. The four experimental diets were formulated containing five GMOS levels of 0.2%, 0.4%, 0.8%, 1.2%, 1.6% feed. Each diet was randomly allocated to triplicate groups of sea cucumber maintained in indoor fiberglass tanks (80 cm × 70 cm) with circulating seawater (18.0± 0.5) and constant aeration. Each tank was stocked with 40 individuals with an initial average weight of 1.79 g ± 0.06 g. The results showed that GMOS supplemented at the experimental levels significantly enhanced the weight growth rate (WGR) and specific growth rate (SGR, <0.05); however, there was no significant difference for ). WGR and SGR peaked at the 1.2% (E4) GMOS supplementation level. In addition, GMOS supplemented at experimental levels had no significant influence on intestinal pepsin, amylase and cellulase content (0.05) or on body wall acid mucopolysaccharide level of juvenile sea cucumber (0.05). sea cucumbers fed a diet with 0.2%–1.2% GMOS had a significantly higher body wall amino acid content, including glutamine, glycine and arginine, than the control (<0.05), while those in the 1.6% GMOS group showed no significant difference from the control (0.05). Furthermore, experimental levels of GMOS supplementation significantly influenced superoxide dismutase (SOD), alkaline phosphatase (AKP) and lysozyme (LZM) activity in the body wall and coelomic fluid (<0.05). In conclusion, sea cucumber fed with a GMOS-supplemented diet not only displayed improved immunity but also an enhanced specific growth rate and amino acid composition of the body wall. Considering all parameters, the optimum level of GMOS in the diet of sea cucumber (1.79 g ± 0.06 g) was 1.2%.