Egg clutches of many animals hatch synchronously due to parental control [1, 2] or environmental stimulation [3, 4]. In contrast, in some animals, embryos actively synchronize their hatching timing with their siblings to facilitate adaptive behavior in sibling groups, such as mass migration [5, 6]. These embryos require synchronization cues that are detectable from eggs and indicative of when the siblings hatch, such as pre-hatching vocalizations in birds and crocodiles [7, 8]. Previous studies, using methods including artificial presentation of non-specific mechanical stimuli, demonstrated that vibrations or other mechanical forces caused by sibling movements are cues used by some turtles and insects [913]. However, there is no evidence about which movements of tiny embryos or hatchlings, among multiple possibilities, can generate mechanical cues actually detectable through eggs. Here, we show that embryos of the brown marmorated stink bug, Halyomorpha halys, synchronize hatching by responding to single pulsed vibrations generated when siblings crack open their eggshells. An egg-cracking vibration seems to be transmitted to distant eggs within a clutch while still maintaining its function as a cue, thus leading to the highly synchronized hatching pattern previously reported [14]. In this species, it is possible that embryos attempt to hatch with short lags after earlier-hatched siblings to avoid egg cannibalism by them [14]. The present study illustrates the diversity of social-information use by animal embryos for success in the sibling group.