Pictorial warnings are effective tools for nudging individuals’ health-related behavior, but much less is known of whether they can also shape pro-environmental decisions. Two online-experiments investigated the effect of simple pictorial warning to reduce the use of plastic packaging. Experiment 1 (N = 289 participants) explored the effect of an environmental pictorial warnings (i.e., a seal suffocating in a plastic bag) on the choice of plastic bags within a fictitious shopping scenario. Experiment 2 (N = 551 participants) investigated whether an environment-unrelated pictorial warning (i.e., a dead fish infested by maggots) would lead to the same effects on plastic bottle choice as an environment-related pictorial (i.e., a dead fish with a plastic bottle in its mouth). Further, environmental attitude and shame were assessed as mediator variables. As expected, the probability of choosing the sustainable product was significantly enhanced in both experiments when participants were presented with a pictorial warning nudge. Experiment 2 revealed that the effect could be assigned to the general aversiveness of the pictures, and not to their plastic-relatedness. The results demonstrate that pictorial warnings work also for pro-environmental decisions, and should be considered in environmental protection campaigns.