Halyomorpha halys (Stål) (Hemiptera: Pentatomidae) is an invasive species in the United States representing a great threat to crops of economic importance, such as soybean and corn. Due to the lack of information about its damage to early vegetative-stage corn, this study was conducted to provide information about H. halys damage to sweet corn seedlings. In the field experiment, caged sweet corn seedlings were exposed to sexed H. halys adults of densities of 0, 1, or 2 insects per plant for 7 d. In a complementary greenhouse experiment, caged sweet corn seedlings were exposed to 0 or 2 nonsexed H. halys at different stages (second to fifth instars and adult) per plant for 14 d. In both experiments, we evaluated plant fresh and dry weights, plant height, stalk diameter and plant injury (using a rating scale, 0 to 5). In the field experiment, plant injury based on the rating scale was greater in plants exposed to insects compared with the control. In the greenhouse experiment, fresh and dry weights, height and diameter of seedlings were lower for those exposed to fourth instars. This stage also caused greater injury based on the rating scale. In general, our results indicate that H. halys can feed on sweet corn seedlings, and that fourth instars cause more injury. The rating scale adapted here can be used for early identification of H. halys occurrence and to assess its injury in the field.