A search for criteria of economically justified and fair taxation has been going on for a long time within studies in philosophy of law, constitutional law and tax law. Often such studies are one-sided and incomplete. The criteria developed by researchers, as well as research-based definition of “economic justification for taxation,” are unsatisfactory and unsuitable for practical use. Without understanding economic justification for taxation, it is impossible to give answers to many practical questions, in particular: is a tax economically justified or not? The purpose of this article is to formulate a new universal, practically applicable definition of “economic justification for taxation,” reflecting modern Russian and foreign ideas about fiscal social contract and constitutional principles of taxation. To achieve this goal, the author resorts to a comparative analysis of two incompatible principles of fair taxation: the equivalence (benefit) principle, arising from “individualistic” Anglo-Saxon theory, and the ability-to-pay principle that is based on continental European “welfare-state” doctrine. The author uses equivalence (benefit) principle to develop a so-called “beneficiary” theory of economic justification for taxation. The beneficiary theory is for the first time set out in this article. The beneficiary theory offers a new understanding of economic justification for taxation which is unconventional for Russian tax law and establishes a clearer – compared to previously known – criterion of economically justified taxation. This criterion makes it possible to identify economically unjustified taxes that do not comply with provisions of tax legislation and Russian Federation Constitution, and therefore can be applied when challenging: 1) validity of legislation establishing economically unjustified taxes; 2) constitutionality of tax legislation. The author, using the beneficiary theory he developed, assesses economic justification for bachelor tax and vehicle tax paid by owner of a vehicle not in use.