The author of this work attempts to reveal and substantiate the fundamental bases of ontology of the sign. The monograph is devoted to philosophical analysis of the nature and essence of the sign, which is substantially disclosed in the concepts of "being" and "matter" of the sign. The concept of "the sign's existence" has been introduced and the criteria for its use have been revealed; the essential property of the sign is its internal transcendence as a condition of its own existence. The sign matter can be understood as an element of sign static, defined by the concepts of name (symbol) and denote (referent). The act of semiose is considered to consist of the fact that a certain entity becomes capable of representing something outside of this entity itself.