This paper deals with the study of the justification of constitutional principles as a fundamental category of constitutional law. Legal principles are an important conception in the legal doctrine and the legal practice of democratic countries. Ukrainian legal doctrine studies legal principles in the two paradigms, namely fundamental principles and general principles. However, this approach does not result in the understanding of principles in constitutional law. That is why principles in constitutional law are an actual topic. The article aims to examine the fundamental concepts to find the justifications of constitutional principles. Hence, the author pays attention to the content of constitutional law and constitutionalism as principles-based categories. First, constitutional law is a fundamental part of the law in the legal system, and all the parts of the law are dependent on constitutional law. Second, the universal constitutional principles are the same for all legal systems. After all, constitutionalism doctrine consists of the limitation of power, and nowadays, this is expressed through the constitution and different constitution principles. The author uses comparative legal, phenomenological, and system-structural analysis as a valid methodology for this research. Finally, the research investigation contains the following conclusions that enable the author to prove the motivation of the science paper to study the outlined topic. First, constitutional law is the basis of the legal system, which embodies constitutional principles; constitutional law has the same effect on public law and private law. Second, constitutionalism is a source for finding constitutional principles. Third, the system of constitutional principles includes the following characteristics: universality and specificity. The system of constitutional principles is open and depends on the interpretation of constitutions. The paper aims to contribute to the growing research highlighting the current issues of constitutional principles. Keywords: principles, constitutional principles, constitutional law, constitutionalism, the system of constitutional principles.