The paper analyses the decisions and stands taken by European Union institutions (the General Court and the European Commission) regarding the issue of whether corporate tax exemptions or reductions for profit generated by the provision of economic activities granted to port managing bodies in Union public ports (port authorities) are State aid. The paper provides a comparative analysis of corporate income tax rules applicable to ports in European Union Member States covered by the decisions of European Union institutions. The Croatian legal framework applicable to seaports open to public transport and their fiscal regime is also presented. The possible consequences of the stances of European Union institutions in relation to the fiscal regime and the functioning of ports in the Republic of Croatia are additionally considered.