The aim of this article is to resolve an issue whether the Article 17 of the European Convention on Human Rights (further referred to as ECHR) contradicts the principle of legality within the judicial practice of the European Court of Human Rights (referred to as ECtHR). The significance of the presented topic does not lie solely within the sphere of academic considerations, but remains of great value for ensuring an adequate level of protection within the Strasbourg system. Moreover, the establishment of the boundaries of implementation of Article 17 ECHR is crucial for providence of legal certainty for all its addressees: individuals, states and the groups of persons. The author of presented paper poses the hypothesis that the manner of practical usage of Article 17 ECHR leads to contradiction of the principle of legality which remains the core for the rule of law concept. The article relies on the legal dogmatic method as well as elements of historic and comparative analysis.