Reducing Emissions from Deforestation and Forest Degradation (REDD) in developing countries may affect domestic policies in related policy fields. Costa Rica has a well-established system of payment for ecosystem services (PES) aimed at protecting forests and biodiversity. Drawing on interviews and a review of the literature, this article examines the possible impacts of domestic and external factors (REDD+ in particular) on the PES scheme in Costa Rica. The analysis builds on theories of domestic environmental policy-making, and of actor participation and interaction at international and domestic levels. In focus are how the legal and institutional system for PES has evolved and how REDD+ might affect this PES framework through diffusion of international ideas and financial leverage of external actors. Domestic development interests and emerging REDD+ principles and methodologies present a combined challenge to the comprehensive view of ecosystem services inherent in PES. Nonetheless, most civil society actors in Costa Rica are strong environmental proponents and seem relatively robust. The legal and institutional framework for PES has proven relatively successful; moreover, compared to most other biodiversity-rich countries, Costa Rica relies more heavily on self-generated funding for conservation and sustainable use of biodiversity.