As the 191 Member States of the International Civil Aviation Organization (ICAO) move to finalize a global market-based measure (MBM) to cap the carbon dioxide emissions of international civil aviation at 2020 levels, interested stakeholders have begun to consider the potential supply of carbon credits that might be available to offset airlines’ emissions above capped levels. This article analyses the potential supply of emissions units from programmes to reduce emissions from deforestation and forest degradation (REDD+) in tropical forest countries, and compares that to the potential demand for such units from international aviation under a global MBM. The article briefly reviews the legal context of the ICAO MBM; summarizes the legal framework for REDD+ established under the UN Framework Convention on Climate Change (UNFCCC) as well as programmes to help tropical countries get ready for REDD+; and notes the underpinnings of provisions relating to the avoidance of double-counting of emissions units. The article then presents estimates of the demand for aviation offsets and the availability of REDD+ units, in the near, medium, and long term, while also considering the need to ensure any of these units used to offset aviation emissions are not double-claimed toward a host country’s emissions reductions plans under the Paris Agreement. The article concludes that REDD+ could provide a significant source of emissions unit supply for international aviation; that an important challenge to realizing this supply is the need for a strong signal of market demand; and that the ICAO MBM has the potential to provide this necessary demand signal.