Under cross-pressure to pledge ambitious emission cuts and deliver concrete policy action, climate policymakers must navigate the tension between ambition and implementation prospects. Achieving the Paris Agreement’s long-term targets is possible only if countries make highly ambitious climate pledges. However, very ambitious pledges might engender widespread implementation failure. Devoid of enforcement mechanisms, the Paris Agreement risks an “ambition trap” whereby policymakers pledge ever more ambitious targets without the willingness or capability to ensure these targets’ implementation. Arguing that the difficulties of implementing highly ambitious pledges might threaten the long-term credibility of international climate cooperation, we report two main empirical findings. First, the ambitiousness of existing nationally determined contributions (NDCs) is inversely related to implementation likelihood, indicating a trade-off between pledges’ ambition and implementation prospects. Second, a conjoint experiment in five major democracies shows that the public is (far) more concerned with emission targets’ implementation likelihood than with their stringency (ambitiousness). Our findings suggest that maintaining the Paris Agreement’s long-term credibility requires aligning NDCs’ ambitiousness with feasible implementation. In short, emission targets must be ambitious, yet realistic.