To accelerate climate change mitigation, substantial lifestyle changes and more ambitious climate policies are urgently needed. However, realizing behavior changes is challenged by widespread misperceptions about the relative climate impact of different behaviors, making it difficult even for motivated individuals to identify which actions to prioritize and which policies to support. Little is known about the cross-national variability of such impact perceptions and their association with climate-relevant outcomes. We aim to address this gap with a pre-registered cross-country study (N = 2742) in China, Germany, and the United States. Using a novel experimental paradigm, we investigated carbon footprint accuracy (how accurately individuals judge the objective size of carbon footprints) and impact perceptions (the perceived contribution of specific behaviors to carbon footprints). We then examined associations between these two constructs and self-reported climate-related behavior and support for behavior-targeted climate policies. Across the three countries, participants showed low accuracy in estimating carbon footprints and underestimated the impact of carbon-intensive behaviors on those footprints. Despite prevalent behavior-specific misperceptions, the impact perceptions of different behaviors were independent of each other, showing no evidence for compensatory judgments. Participants’ carbon footprint accuracy and impact perceptions were associated with corresponding self-reported behavior across all countries. However, the association between impact perceptions and climate policy support varied across countries, with impact perceptions having a direct effect in Germany, no effect in China, and a moderated effect by political orientation in the United States. Our study highlights the need for cross-national research to further uncover which contexts foster (in)accurate behavioral knowledge and corresponding climate-friendly behavior.