The focus of the article is on two large-scale transformation processes that are currently taking place in the Western scientific and socio-cultural spaces. On the one hand, it is the emergence on the cultural horizon of a new macrocultural era, called Metamodern, on the other hand — the transformation in the paradigm field of Western science and the emergence of a paradigm of complexity (complexity theory). The statement is argued, that they are the different manifestations of the general large-scale socio-cultural and scientific tectonics and can serve as contours for mutual legitimacy and justification. The metamodern project captures the emergence of a new worldview in Western culture, which is characterized by a new sincerity, openness, holism in contrast to postmodern alienation, irony and sсepticism. In terms of experiencing the I-World relationship, the logic of cultural progress from Tradition to Metamodern can be described as a movement from complete immersion of the Self in the World (Tradition), through confrontation and struggle between the Self and the World (Modern), chaos of the I (multiple identity) and the World (current modernity) in the postmodern era, to a new dialectical combination of the I and the World in the Metamodern era. Complexity Theory in its sociological theoretical proposals can offer a parallel view on the process of changing cultural epochs, but through the prism of transforming the dominant mechanism of change and maintenance of social order, emphasizing the existence of two such mechanisms — organizational one and self-organizational one. The logic of changes in this dominance leads us from Tradition (self-organization), through Modern (organization), and Postmodern (chaos, no domination, the mutual distancing) to Metamodern — dialectics, tense unity of organization and self-organization. In socio-cultural terms, it looks like a dialectic of modernity (rationality, purposefulness, result, organization) and tradition (myth, game, process, self-organization), and at the level of basic experience of the world as a dialectic of acute experience of one's self combined with the experience of its unity with the world. It is at this point that the metamodern project and the paradigm of complexity in its sociological interpretation do meet. Here, they can give each other legitimizing support.