AbstractSarcopenia and obesity are emerging as major social problems. In this study, we examined whetherGryllus bimaculatus(GB), an edible insect, prevents dexamethasone‐induced muscle atrophy (sarcopenia) or high‐fat diet (HFD)‐induced obesity in mice. We generated a standard chow diet (SCD) + GB (85% SCD and 15% GB powder) and HFD + GB (85% HFD and 15% GB powder). SCD + GB feeding increased gains in body weight and white adipose tissue (WAT). Despite no difference in weight change between HFD + GB‐ and HFD‐fed mice, HFD + GB feeding aggravated insulin resistance compared with HFD feeding. SCD + GB or HFD + GB feeding did not change most gene expressions in the liver and WAT but did increase MyHC1 expression in the muscle, meaning that GB increased muscle generation. Therefore, we fed SCD + GB with dexamethasone, which induces muscle degeneration. As a result, muscle fiber size increased, as did grip strength compared with dexamethasone‐injected mice. In addition, SCD + GB reduced the expression of muscle degradation factors, such as atrogin1 and muscle RING‐finger protein 1 (MuRF1). Furthermore, SCD + GB feeding increased Akt, mTOR, and p70S6K phosphorylation and MyHC1 expression, meaning that it may have increased protein synthesis. In conclusion, GB has great potential for inhibiting dexamethasone‐induced muscle mass loss by increasing muscle protein synthesis and inhibiting muscle protein degradation.