The paper aims to assess changes in the level of competition in Poland’s banking sector in 1997-2007. Competition between banks is one of the most important factors behind the stability of the financial sector through its influence on the profitability of banks, access to external funding, and the country’s economic development as a whole. In this paper, the Panzar and Rosse model was applied to assess the level of competition in the banking sector. The results of the analysis of the Polish banking sector show that between 1997 and 2007 commercial banks operated under monopolistic competition. The degree of competition in the Polish banking sector is close to that in euro-area banking sectors (which is reflected by the values of the Panzar and Rosse measures). This applies to both corporate and retail banking. The main driving factor behind competition in the Polish banking sector was the country’s entry into the European Union. The same channels, i.e. consolidation and financial deregulation, that were observed in the EU at the time of adopting the euro influenced competition between banks in the Polish banking sector at the time of EU entry. This was mainly due to foreign capital from the euro area.