A strong environmental self-identity increases the likelihood of a wide range of proenvironmental actions. But which factors influence identity and can we strengthen it? We propose that the environmental self-identity depends on biospheric values and on past behavior and that the strength of one’s environmental self-identity can be changed somewhat by reminding people of their past environmental behavior. We tested our model in a series of studies and show that biospheric values and past environmental behavior influence the environmental self-identity, which is in turn related to subsequent environmental judgments and intentions. Furthermore, we found that although the strength of the environmental self-identity changed when we reminded people of their past environmental actions, biospheric values remained an important predictor of self-identity, suggesting that the environmental self-identity has a stable core. Our results further suggest that environmental-friendly behavior can be promoted by reminding people of their past proenvironmental actions as this will strengthen one’s environmental self-identity.