In this paper, an aggregate indicator of a regional green economy (Regional Green Economy Index-RGEI) was proposed and applied to assess the level of green economy in Polish regions and its changes in the period 2004-2016. The TOPSIS (Technique for Order of Preference by Similarity to Ideal Solution) method was applied, which is one of the multi-criteria decision making methods (MCDM), widely used to assess the spatial diversity of socio-economic phenomena. Common reference values (ideal and anti-ideal solution) were used for variables for the entire study period. It allowed not only for creating a ranking of regions, but to assess progress towards the green economy as well. It was found that all regions of Poland made progress in this respect. Most importantly, the regions ranked the worst before Poland's accession to the European Union, made substantial progress. It was stated as well that none among the studied regions had high values of all variables included in the aggregate index. The maximum value of the RGEI index was about 0.5, while the index range is [0,1]. Additionally, an important finding was the fact that the weights of all diagnostic variables obtained using information entropy method were about equal, which confirms the approach of researchers and institutions who do not use weighting in aggregate indicators for well-being or sustainable development-which means implicitly using equal weights.