Abstract This article addresses the legal aspects of the financing of adaptation to climate change, with special consideration given to one of the climate funds – the Adaptation Fund (AF). In the complex structure of climate finance, the AF attracts attention as it differs from other funds in several aspects. As an exception from other United Nations (UNs) climate funds, AF has not been recognised as an operating entity of the Financial Mechanism of the Convention. AF is also an unprecedented example under the Convention of a fund serving in parallel two decision-making bodies (CMP and CMA) under two agreements (Kyoto Protocol and Paris Agreement) ratified by non-overlapping sets of parties; with a Fund Board elected by both bodies collectively. AFs funding source is specific, it was designed to be financed from shares of the proceeds of sales of certified emission reduction units (CERs) generated by Clean Development Mechanism (CDM) projects but since 2012 there is a limited possibility of offsetting the emissions with CERs under the EU emissions trading system (ETS). For several years Parties were unable to agree the operating principles and procedures of a new mechanism established in article 6(4) of the Paris Agreement, which will resemble CDM, and would constitute a source of funding for the AF. Once this source of funding is available, the AF would cease to serve the Kyoto Protocol. Despite the above problems, AF was seen as a good example of how future climate funding can be designed. We examine the evolution of the above legal problems, including the most recent decisions and conclusions adopted during Conference of Parties (COP)25 in Madrid.