Green finance plays a critical role for achieving sustainable development goals. Scaling up green investments and increasing their efficiency requires addressing a number of methodological and practical problems. One of these is a problem of efficiency evaluation. The approach to assessing green investments efficiency proposed in the paper is based on the investment portfolio model. Its application makes it possible to assess green investments with regard for their principal impact on the investment portfolio quality rather than on the basis of actual results of environmental projects. The proposed methodology makes it possible to derive an acceptable rate of return on green investments for inclusion in the investment portfolio, as grounded on the identification of alternative ways to achieve environmental objectives. Using the example of forest cultivation, an algorithm is presented for estimating the guaranteed return proceeding from natural productivity, which can be used to evaluate the acceptable efficiency of investment in both forestry and alternative projects that aim at reduction of greenhouse gas concentration and development of sustainable power engineering. Even in case of low return, green investments can be financially attractive if they contribute to reducing the investment portfolio risks. The usefulness of the proposed approach depends on completeness of accounting the investment projects’ environmental risk in overall market risk evaluation.