Annually, millions of tons of plastic waste end up in rivers and the oceans, obstructing the surrounding ecosystems and resulting in substantial environmental degradation, as well as unforeseeable negative consequences for several species, including humans. In tackling this plastics crisis, the United Nations Environmental Assembly has decided to draft international legally binding instruments targeting the manufacture and use of problematic types of plastic waste. The implementation of plastic policies is considered a key component in tackling the ongoing plastic crisis and, with this, prevent future catastrophic consequences on the ecosystem caused by plastic pollution. Nevertheless, people's willingness to accept these policies is paramount in achieving the intended goal. In this study, we examined social norms, institutional trust, and generalized trust as predictors of the acceptability of plastic policies in Sweden (N = 2006) and Brazil (N = 2002). Multiple hierarchical regression analyses showed that institutional trust, generalized trust, and social norms positively predicted acceptance in both countries. Consistent across countries, we found that social norms explained a noticeably larger proportion of the variance in acceptability than both institutional trust and generalized trust. These results advance past research on policy acceptability by a) assessing acceptability of a plastic policy, b) providing cross-cultural data, and c) showing that the well-studied determinant trust was less influential than the less-studied determinant social norms. These results stress the role of social norms, both for future research and for practical applications, encouraging further study on how social norms can be implemented to encourage acceptability of environmental policies.