The sharp increase in and volatility of fossil fuel prices, due in particular to the Russian–Ukrainian conflict, is a powerful incentive for cities to accelerate their energy transition. Yet urban authorities have limited power over the construction of energy policies and the management of networks, and they remain dependent on remote and mainly carbon-intensive imported sources of energy. The recovery of waste heat from waste incineration or industrial emissions and its use in heating networks represents a solution for cities to control part of their energy supply, to develop their own capacities for action and to implement local transition strategies, in addition to the development of renewable energies. Based on the analysis of four case studies in France between 2019 and 2022, in the context preceding the current energy crisis, this article examines how cities are trying to develop waste heat recovery and the role this energy resource plays in the decarbonisation of urban energy systems. The analysis highlights that the emergence of these projects is more broadly part of the renegotiation dynamics of energy, ecological and economic relationships between cities and industries, and that their implementation results in the construction of new urban energy nexuses. The use of waste heat makes it possible to improve the energy efficiency of industrial and urban energy systems, sometimes significantly, but it must be seen as a transitional solution because it can temporarily increase cities’ dependency on high-carbon and energy-inefficient industrial activities.