Public engagement in pro-environmental behavior and support for pro-environmental policy are essential for achieving sustainable living. We propose that the “moral circle” is a common motivational source for engagement in environmentally beneficial activities across situations and may be thus drawn upon to efficiently promote these activities. Study 1 established an association between chronic moral circle size and nine pro-environmental activities from different domains. Via experimental manipulation of the moral circle size, Studies 2a–d demonstrated its causal effect on intentions to engage in pro-environmental activities. Together, these studies offer an important initial demonstration of the beneficial consequences of more expansive moral circle in the domain of pro-environmentalism. Routes for expanding the moral circle and thus promoting pro-environmental activities are discussed.