The paper presents the conception of the Mythological Dictionary of the Altaians, the principles of description and thematic classification of mytholexemes. The conception of the mythological dictionary is outlined. The dictionary’s development takes into account the specifics of the national worldview. The purpose and objectives of the dictionary are formulated; the material and research methods are characterized; the description and characteristics of the structure of the dictionary and dictionary entry are given; the basic principles of glossary formation are highlighted. The compiled dictionary refers to the linguoculturological explanatory dictionary type. The main principles for choosing mythemes were: (1) complete or partial inconsistency of the mythological significate (the conceptual content of a name or sign) with the referent (an object of extralinguistic reality); (2) mythologism, or the presence in the semantics of the word of a reduced myth; (3) conceptual content that is not limited to the lexical meaning of the word, and the semantics of the mythological concept revealed by its historical, ethnographic, figurative, evaluative components; (4) stability and variability of the mytholexeme in the mythology and folklore of a given ethnic group or area. The identified mytholexemes were classified based on their part-of-speech affiliation, word-formation pattern, and conceptual content. In addition to the mythological vocabulary of the southern Altaians (Altai-Kizhi, Telengits, Teleuts), the dictionary includes the vocabulary of the northern dialect groups (Chalkans, Kumandins, Tubalars). The dictionary is illustrated by contexts from published and unpublished sources, archival and field materials, texts of different genres that contain myths, legends, mythological stories, legends, heroic tales, and ritual texts of the Altaians. The dictionary has thematic sections: “The World of Deities and Spirits”, “The World of Nature”, “The World of Man”, “The World of Things”. The thematic section “The World of Deities and Spirits” includes subsections: deities and spirits of the Upper World, deities and spirits of the Lower World, deities and spirits of the Middle World, the world tree. The section “The World of Nature” consists of subsections: planets and stars, natural phenomena, metals and minerals, flora and fauna. The section “The World of Man” includes subsections: man and his soul, parts of the body, heroes, social roles; birth; time and calendar; cult objects; food; ritual actions. The section “The World of Things” contains subsections: buildings and household items; weapon; clothing and jewelry; cultural items. A special subsection contains lexemes expressing numerical and color features of objects. The bilingual explanatory Mythological Dictionary of the Altaians acquaints the reader with the mythological concepts, ideas of this ethnic group that go deep into their ancient pagan beliefs and shamanism. It can be in demand not only in studies of the mythology and beliefs of the Altaians, but also among a wide circle of readers interested in the culture of the Turkic and other peoples of Siberia. The author declares no conflicts of interests.