Across a series of six high-powered studies (N=4,878), we develop and validate the Longtermism Beliefs Scale (LBS) to measure alignment with the “longtermism” philosophy, which advocates for the welfare of distant future generations. The findings highlight the pivotal role of the LBS in predicting attitudes and behaviors aimed at protecting future generations from extinction threat (Studies 1-5). The LBS reveals significant associations with heightened concern for human extinction threats and negative appraisals of future extinction, even when contemplating distant hypothetical futures (Studies 1-3). Notably, higher longtermism beliefs are linked to perceptions that future threats can be resolved, support for policies seeking to protect future generations of people, and a profound sense of responsibility for the long-term survival and prosperity of humanity (Study 4a). The LBS also proves instrumental in predicting various future-oriented attitudes, patterns of prosociality, and longtermism-related behavioral intentions (Studies 4b-5), and is capable of meaningfully distinguishing between self-identified longtermists and non-longtermists. Overall, this research contributes to the field by introducing and validating the LBS as a critical tool for assessing longtermism beliefs and understanding their role in addressing existential threats to the long-term future of humanity.