Contributing a new South American case study, this paper seeks to advance the research agenda on processes of policy integration by developing a better understanding of how nascent subsystems become integrated into mature ones and the role that changing beliefs of advocacy coalitions play in fostering policy integration. The paper examines environmental policy integration in Uruguay’s forestry sector since the 1990s and is based on an inductive qualitative analysis of policy documents, sector reports, parliament hearings and semi-structured interviews with key stakeholders. This demonstrates that environmental policy integration has increased continuously since the 1990s, accelerating particularly during the 2000s. We can derive three insights that specifically address this path of integration: a change in the policy beliefs of the dominant advocacy coalition, international salience of the minority coalition`s beliefs and participatory policy processes that foster interactions between opposing coalitions. Despite this, the two advocacy coalitions have crystallized with fundamentally different deep core beliefs about what a sustainable forestry sector should be. While one coalition argues that commercial tree plantations are sufficiently regulated in environmental terms, the other coalition maintains that the way that the pulp industry has developed in Uruguay is fundamentally unsustainable and therefore seeks to change the forestry sector as a whole.