In current debates about the Anthropocene, the notion of ‘technosphere’ has gained analytical traction. It is loosely defined as the conjunction of all technological systems embodied in artefacts that have been created by humans since the domestication of fire and the invention of the first tools. Authors such as Peter Haff argue that the technosphere can and should be investigated as a physical phenomenon. We agree but raise the question of how those features can be accounted for that are often conceived of as specifically human, such as culture, agency, consciousness and creativity. We suggest that technosphere science needs to include semiotics: All physical interactions in the technosphere are mediated via physical signs, and signs also mediate human action. This requires a fundamental rethinking of our common conceptions of doing science; especially, we advocate a richer conceptualisation of causality. We build on two classical approaches to semiotics: C.S. Peirce’s semiotics, as further developed in modern biosemiotics and Yuri Lotman’s notion of semiosphere. We posit the principle of ‘bimodality’, where technosphere interactions are always and everywhere in the two modes of matter-energy transformations and semiosis. In this framework, we suggest that the economy is a core constituent of the technosphere, mediating between physical processes and human agency. To pursue the implications of our approach, we suggest that research into the phenomenon of the city and urbanisation is a central concern of semiotic analysis of the technosphere. In the evolution of urban systems, physical aspects (such as the evolution of material networks and physical flows) always work together with semiotic aspects of social networks, to produce the semiosphere of a city.