Certain behaviors, especially when adopted by a majority, can help limit the magnitude of climate change. In this cross-sectional study, the stages of change from the Transtheoretical Model (TTM) are applied to environmentally-significant behavior. We develop and test a new scale -- the Climate Change Stages of Change Questionnaire (CCSOCQ) -- for measuring five discrete stages involved in changing behavior to address climate change (precontemplation, contemplation, preparation, action, and maintenance). In total, 3,469 middle-school students (M[subscript age] = 13.4) and 347 schoolteachers completed the CCSOCQ and a measure of engagement with sustainable development. Analyses indicated the CCSOCQ captured the five stages outlined by the TTM and had adequate psychometric properties. Further supporting predictions of the TTM, engagement with sustainable development was lowest in the precontemplation stage and highest in the action stage. We conclude the TTM has potential for describing how and why people alter their behavior to address climate change. Supplemental data for this article is available online at https://doi.org/10.1080/13504622.2021.1998382 .