In light of the organizational dynamics of services of economic interest, the regulation of municipal solid waste management is a critical issue to deal with so as to achieve sustainability goals in the coming decades. The European circular economy targets limit the share of municipal waste in landfills to a maximum of 10% by 2035. Consequently, waste-to-energy plants may temporarily become the primary option for residual unsorted waste. The municipal waste management chain comprises two consequential stages: collection and transport, and the treatment and disposal stage, which characterizes as an oligopolistic market structure. After defining the relevant market and calculating market concentration measures, we analyze market power in the treatment and disposal of non-recyclable mixed waste, also known as residual waste. Our analyses are based on empirical data using well-known market concentration indices such as the Herfindahl–Hirschman index and concentration ratios. We report the results of three different market concentration scenarios based on alternative geographic and product market definitions. Considering only waste-to-energy as a product market, we present a situation of moderate concentration, typically involving the attention of competition authorities. On the contrary, considering both options as a single product market, no relevant evidence emerges due to the significant share of waste sent to landfills in 2019, i.e., 20.1% of the total municipal solid waste generated in Italy. Implications for future studies consist of new detailed information on the municipal waste treatment market structure in one of the leading European countries that may prompt comparative studies. Policy implications are derived from the possibility of taking cues from this paper to envisage appropriate regulatory models for an evolving sector in which market spaces are increasing.