One of the main issues of implementation of the concept of sustainable development is the financing of relevant projects, so in modern conditions, along with the three basic elements – economic, environmental and sociological – is also allocated the financial element. The development of the «green economy» has led to the emergence of «green finance». However, there is no single established understanding of the definition of «green finance», thus analyzing existing approaches is the purpose of this research. As a result of the analysis the key approaches to understanding «green finance» are systematized as follows: 1) process; 2) financial investments in various forms of manifestation; 3) financial flows; 4) sustainable financial system. The last approach is determined as the most system-related. The definition of «green finance» via financial system is provided. The subject-object characterization of the financial system of sustainable development, which includes regulators of direct and indirect influence, and also objects, subjects (investors and recipients) is presented. The subjects and objects are coordinated with the spheres and links of financial system. The obtained results become the basis for further studies on systematization of specific financial instruments and their characteristics.