Egg cannibalism is a widely observed behaviour in nature that provides nutritional benefits without the costs of subduing prey. However, when egg cannibalism involves sibling eggs, the benefits obtained must balance inclusive fitness losses. Neonates of the Colorado potato beetle, Leptinotarsa decemlineata, readily consume other eggs within their natal clutch. To explore the fitness consequences of this behaviour, neonates were fed no, one, or two eggs upon hatching, along with their normal diet of potato foliage. Daily mass, the time to each moult, adult volume and mass, and fecundity were measured for each individual, along with mortality rates. Cannibals gained mass more quickly and reached each developmental stage 1 day earlier than noncannibals. No benefits at the adult stage (other than earlier emergence) were observed, and there were no sex differences in the benefits of cannibalism. Mortality was not reduced by cannibalism under laboratory conditions. A simulation demonstrated that when the mortality risk is high, decreased development time due to cannibalism can reduce this risk sufficiently to balance the inclusive fitness loss of eating a single half-sibling. Additional benefits of cannibalism, such as reduced competition, need to be investigated to fully explain cannibalism of multiple sibling eggs. ► Beetle neonates fed eggs gained mass more quickly than those fed only foliage. ► Cannibals reached each developmental stage at least 1 day faster than noncannibals. ► No sex differences were observed in the benefits of cannibalism. ► The observed development-time benefits compensate for the death of a half-sibling.