<p>Environmental issues such as climate change have become major contemporary concerns that affect beliefs and, in some cases, behaviors. We examine whether and how environmentalism may have contributed to the second demographic transition that has driven declining fertility in some parts of the world since the 1970s. Data come from the 2006–2014 and 2016–2020 General Social Survey rolling panels in the USA. We fit Bayesian linear mixed-effects models to estimate the effects of ones’ environmental support, as measured by their support for the government’s environmental spending, on their actual childbearing and beliefs about ideal family size. We found that ones’ environmental support was negatively associated with both outcomes. Specifically, the negative association between environmental support and childbearing was stronger among younger cohorts. Among reproductive-aged women (ages 18–45), their environmental support was negatively associated with the likelihood of new childbearing contemporaneously, but did not predict future childbearing. These findings suggest that environmentalism has largely contributed to the second demographic transition in the USA, while also raising another mechanism that parents may deprioritize the environment to cope with expected childrearing responsibilities.</p>