The aim of this article is to identify an optimal method for estimating gross domestic product at the regional level in Poland, specifically at level of counties. While various studies offer empirical analyses of the country’s economic growth and other economic processes, no official data is available on GDP at the level of counties in Poland. The article presents the results of research based on three alternative approaches that rely on GDP disaggregation from the level of sub-regions (NUTS-3) to the level of counties (NUTS-4). These methods have been validated by comparing the results obtained for the higher level of aggregation. It turned out that the best method of disaggregating GDP was to use the shares of municipality tax revenues from income tax and agricultural tax. The estimated values of GDP in the counties in 2003–2013 were used to calculate the rate of economic growth in the entire period as well as in the period before and after the global economic crisis. We also examined the spatial autocorrelation of GDP per capita in 2013. The time series of GDP at the county level in both constant and current prices generated as a result of the presented research can be used in other empirical analyses.